This guide is probably obsolete, use this instead. Just drag your otp.bin to your browser window and it will compile it for you, I'm leaving the original guide below for those who likes to DIY.

Original post:
Hopefully this guide should silence the whiners

Before we begin I have to say this is my very first guide that I made from my experience and information from other Temp members, and English isn't my primary language so I might make mistakes. If you find any just tell me and I'll try my best to correct them. (The screenshot in the spoiler was taken in OS X with wineskin because I'm lazy, this guide is for Windows only)

So you just dumped your OTP and wondering how do you compile A9LH for your 3DS? Here's a guide to instruct you how to compile and install the payload on your device. This guide assumes you know the basics of 3DS homebrew and Windows command prompt,and just stay away from this if you don't - menuhax is good enough for most people IMO. (I am serious)

DISCLAIMER: Any modifications to your 3DS are done at your own risk. I am not responsible for any bricks that may occur during this process (use your common sense and read the guide carefully)

0. Requirements:

• arm9loaderhax source (https://github.com/capito27/arm9loaderhax), it includes all the file dumps you'll need, except your own OTP
• miniPasta (https://github.com/d0k3/MiniPasta/releases , N3DS only)
• OTP.bin (DO NOT use dumps that are not from your console or the one included with the A9LH source, you WILL brick your 3DS, guaranteed.)
• Python 2.7.11 x86-64 (https://www.python.org/downloads/windows)
• PyCrypto (http://www.voidspace.org.uk/python/pycrypto-2.6.1/pycrypto-2.6.1.win-amd64-py2.7.exe)
• Make(http://gnuwin32.sourceforge.net/packages/make.htm)
• devkitARM (https://sourceforge.net/projects/devkitpro/files/Automated Installer/ , part of devkitPro)
• Your brain

1. Preparation

1. If you have an existing install of Python 3.x/32-bit 2.7.x, remove it first
2. Install Python 2.7 x86-64, make sure you install "add python.exe to patch" sub feature during installation (it's not set by default)
   
   Spoiler

   
3. Install PyCrypto
4. Install devkitPro 1.6.0, you will be be presented with tons of packages but the only ones we're interested are under devkitARM, simply uncheck everything else.
   
   Spoiler

   
5. Install Make, you should download the file with md5:8ae51379d1f3eef8360df4e674f17d6d

2. Compile A9LH

1. Extract the A9LH source to any directory
2. Copy your OTP.bin to the data_input folder and overwrite the one that's included with the zip file
3. Open a CMD window (Shift+right click on the explorer window or cd into it manually)at the root directory of the source files, type "make" (without quotes) and enter.
4. Wait for a minute or so, a data_output folder will appear and there should be no errors
5. Copy the arm9loaderhax.3dsx file from the newly created folder to /3ds/ on your SD card (the installer is built specifically for the 3DS you dumped its OTP from, if you want to install A9LH on more than one 3DS you will to recompile the payload with their respective OTP.bin files)

3.1 Installing A9LH

1. Ensure you have a dump of your SysNAND before you continue
2. Enter hblauncher using an entry point of your choice
3. Run arm9loaderhax.3dsx (see below if you're stuck on "exploiting arm9")
4. The payload will be installed to your NAND and your 3DS will then be rebooted to black screens afterwards. Don't panic, turn off your 3DS

3.2 "exploiting arm9" workaround (required for n3DS, not sure about o3DS)

1. Download and copy the miniPasta 3dsx to your SD card
2. Run miniPasta, the screen will turn black for a few seconds and you will be kicked back to home menu afterwards
3. Re-enter hblauncher and try running the A9LH installer again

4. Testing the payload + conclusion
There's a test binary (forgot who made it) that will power off your 3DS upon pressing any buttons except ZL, ZR or home. Simply copy it to the root of your SD card, wait for a couple seconds and press a button. If your 3DS powers off, then A9LH is successfully installed and woking correctly (congrats). If your 3DS powers off the moment you turn it on, check if the file is named "arm9loaderhax.bin" and placed in the root directory, if it power led stay on and won't respond to button pressed then your 3DS is probably bricked
If you want a cold boot (permanent) CFW (the reason to get A9LH in the first place), check out @Aurora Wright's ReiNand fork. She already documented it very well so I'm not going through it in this thread

FAQ
Q: I'm getting "python: command not found"
A: Make sure you select "add python.exe to path" during installation, alternatively you can set the PATH variable manually by entering this in cmd (Thanks @Kever)

set PATH=C:\Python27;%PATH%

Q: How to uninstall A9LH?[/SIZE][/SIZE]
A: Restore a sysNAND backup made before running arm9loaderhax.3dsx

 

[Kever]

Interesting.

I don't have a hardmod, so I will not test it. But if it's true, if it's really working just like that, it simplifies a lot the swap procedure.

y i forget to delete the nand.bin before install (sysnandbackup) and it installs without problem (but dont know how risky it is)
now i have aureinand ... but dont know how to update a9hl if there is a newer version. Same to injecting emunand after install a9hl. Because of different encryption of emu-and sysnand.
Any Guides without reinstall a9hl? Perhaps decrypt and import?

 

[max_505]

AuReinand use rei folder too
No need Reinand.dat only firmware.bin inside rei folder and arm9loaderhax.bin on root sd card

That made it work. Thank you very much! And thanks to jon2491 for helping me to confirm that my MD5 values for the arm9loaderhax.bin were correct!

 

[Halo249]

I install python when installing pycrypto it says "Can't find Python 27"

 

[TheDuckMan64]

I install python when installing pycrypto it says "Can't find Python 27"

Have you installed Python 2.7 (Not Python 3.x)? If so, make sure the installer is pointing towards python's directory when you install.

 

[Daidouji]

So, after realizing that I had reiNAND and not AuReiNand installed and almost depressing thinking I bricked my console... it works! =D "search only on this thread" option in the search box has to be the best damn thing ever! xD

I lost all my "custom" stuff... A.K.A. ctrlBoot, menuHax and reiSplash images... but this thing boots twice or even trice faster than with the previous setup (menuhax + coldboot) and it has 100% success rate! And I love that! Because I tend to turn off the console when not it in use for a long period of time and always left a game cartridge in the console and that sometimes effed up the boot xD

Also turning on the console while closed boots 100% of the time! I love this! =3

 

[Halo249]

Have you installed Python 2.7 (Not Python 3.x)? If so, make sure the installer is pointing towards python's directory when you install.

How?

 

[TheDuckMan64]

How?

How what?

 

[Halo249]

How what?

NVM Python just isn't working with me.

 

[TheDuckMan64]

NVM Python just isn't working with me.

Pm me your otp.bin and i can compile it for you if you want.

 

[conradcervantes]

Anyone mind compiling mine? I have all the necessary programs installed but cmd isn't cooperating with me.

 

[Sachi]

Anyone mind compiling mine? I have all the necessary programs installed but cmd isn't cooperating with me.

if it's n3ds pm me

 

[conradcervantes]

^Darn, it's my o3DS.

 

[Sachi]

^Darn, it's my o3DS.

Check if the o3ds and n3ds are the same. If so I could still do it. I think it should be

 

[TheDuckMan64]

Check if the o3ds and n3ds are the same. If so I could still do it. I think it should be

I was wondering the same thing actually.

 

[bache]

Never mind, I fixed it.

 

[piratesephiroth]

by the way that dumb PyCrypto installer didn't work for me (said it couldn't find Python 2.7 in the registry)

So I recommend installing it with the command line

easy_install http://www.voidspace.org.uk/python/pycrypto-2.6.1/pycrypto-2.6.1.win32-py2.7.exe

 

[Revax]

I install python when installing pycrypto it says "Can't find Python 27"

I had this issue because I accidentally downloaded and installed the 32 bit version (you can check what version you installed by running Python).

Just make sure that PyCrypto and Python are the same bit version

 

[piratesephiroth]

I had this issue because I accidentally downloaded and installed the 32 bit version (you can check what version you installed by running Python).

Just make sure that PyCrypto and Python are the same bit version

I didn't even know there was a 64bit version. I don't even know why it is recommended in the guide instead of the x86 version, since it's not even necessary for this.

 

[Revax]

I didn't even know there was a 64bit version. I don't even know why it is recommended in the guide instead of the x86 version, since it's not even necessary for this.

I think the PyCrypto provided is 64 bit, if anyone does want the 64 bit version of Python, this worked for me - https://www.python.org/downloads/windows/

 

[Koryushin]

I had this issue because I accidentally downloaded and installed the 32 bit version (you can check what version you installed by running Python).

Just make sure that PyCrypto and Python are the same bit version

I had this issue too and once i get the good one, i forgot to re-add c:\Python27; on my environment path