This guide is now outdated.
You can now install the DSi Homebrew Menu as a CIA here:
http://gbatemp.net/threads/dsi-homebrew-menu-directly-on-your-home-menu.416791/
http://gbatemp.net/threads/dsi-homebrew-menu-directly-on-your-home-menu.416791/

Note that you must have patched TWL_FIRM before it can run, look here for a guide on that topic:

http://gbatemp.net/threads/tutorial-patch-twl_firm-to-play-normally-blocked-ds-games.414578/

The old guide is below for those who still wish to install sudokuhax.

Spoiler

This is a tutorial that shows an easier way to install the DSi exploit 'SudokuHax' onto a 3ds console. Previous guides on this topic have involved a lot of steps which may not be easy to the noob who wants DSi homebrew on his/her 3ds. Due to recent advancements in hacking TWL_FIRM, an easier method has been opened up that doesn't require manually injecting Sudoku into your Nand. Without further ado, lets get started.

Step 1: Install a Patched TWL_Firm into EmuNand
Search a certain well known site for a thread titled: "Patched TWL_FIRM cias for O3DS and N3DS" and install the respective CIA file. Sorry but I cannot link it here. Alternatively, use a recent nightly build of RXTools, it has the patches built in.

For an easy walkthrough on this step, perform steps 0-3 from this guide.

This patches the signature check that the 3ds uses to verify DSi games when it runs them.

Step 2: Install Sudoku
Search the same site as above for the thread titled: "DSiWare Mega Pack. 112 DSi Games for your 3DS!"
Download the file and extract the cia titled SUDOKU-Electronic_Arts_Inc..cia. Drag it onto your sd card and install it onto both Sysnand and Emunand with the cia manager of your choice. Make sure the file gets installed to Nand on both, not the SD Card.

This will install the actual game which has the exploit. Sorry but I cannot link this file either.

Step 3: Install and Load TWLNandFiler
Search the same site again (noticing a pattern?) for the thread titled: "TWLNandFiler and WMTestTool (Retail patched)" and download the file called TWLNandFiler_RetailPatched.rar. Extract the file with .cia on the end, and drag it onto your sd card.

Download the save file attached to this post, extract it, and put it onto your sd card as well.

Install TWLNandFiler onto both EmuNand and SysNand with the cia manager of your choice, and then launch it through the Cia manager. (For FBI launch the title called "NandFiler" while your destination is Nand, while in DevMenu/BBB launch the program called "TWL NandFiler).

This will load into an application that allows us to manipulate the save files of DSiWare installed onto the 3ds.

Step 4: Install the Save Exploit
When you have entered TWL_NandFiler, scroll down to the yellow folder titled "sd", press A to select it, then

find the file you put onto your sd card in step 3 called K4DE_public.sav. When your selector is on it, press X to bring up the action menu, and click on the restore option. Press A to confirm.

This will install the actual exploit into the Sudoku save.

Step 5: Running the DS Homebrew Channel
Download this file:

https://gbatemp.net/attachments/boot-zip.21417/

and extract it onto the root of your sd card. This is the homebrew launcher that is commonly used to launch dsi homebrew. Test out your newly installed Sudokuhax by loading the Sudoku game and moving past the main menu. When the HBMenu loads you will have a fully functioning Sudokuhax on your 3ds, and you will be able to launch DS/DSi Homebrew that you can place on your sd card!

Credits to
@TuxSH and @Steveice10 for the TWL_FIRM Patches
@Apache Thunder for getting TWLNandFiler running on non-dev consoles, among other things in this guide
Team Twiizers for the SudokuHax exploit
@ahezard for editing the SudokuHax save file so that it would work in TWLNandFiler
@zoogie for releasing this modified version of the DS Homebrew Launcher that works with SD Cards higher than 2 GB

 

[Mazamin]

nope.

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i think

Why do you think this? It's not alreaady been developed, but this doesn't mean it's impossible

 

[Deleted member 373223]

Why do you think this? It's not alreaady been developed, but this doesn't mean it's impossible

but nobod will develop it, peoples are too busy creating a dsiware app of wood dumper

 

[tivu100]

I never have a dsi so I kinda missed what is so cool about DSI homebrew that people working on getting Sudokuhax onto 3DS.

So what the beinifit of Sudokuhax for 3DS (what I know so far is install/ changing ROP)? What else?

 

[1ted59]

The guide has been updated to reflect the fact that there are better methods to run DSi Homebrew on the 3DS.

 

[Antonio2311]

As you said, this is useless since now we have a better option to launch DS/DSi homebrew, but it's still pretty cool to have this classic exploit on the 3DS as well.

 

[DRAGONBALLVINTAGE]

Could this work on a DSi Since we have a way to inject DSiware now?