I can't seem to boot into it. Select boots me in ReiNand config, RB attempts to boot me into emuNand(which doesn't work because I don't use one), LB attempts to do some funny business with ReiNand and firmware, and the d-pad and face buttons don't do anything. Holding start just gives me a normal boot. Thoughts?
If you don't want to mess with changing any settings or changing your ReiNAND, you can just do this:
Download the decrypt9 archive
Rename arm9loaderhax.bin on your SD card to arm9loaderhaxold.bin
Copy arm9loaderhax.bin from the decrypt9 archive to your SD card
Boot your 3ds and you'll go straight to decrypt9
Do what you need to do in decrypt9
Power off, rename arm9loaderhax.bin to decrypt9.bin, and arm9loaderhaxold.bin to arm9loaderhax.bin
As others have said though, changing to Luma will allow you to have a boot menu when you start up so you can quickly boot to other a9lh payloads like decrypt9 by holding down keys on boot. But the above might be quicker and easier if you just want to run decrypt9 as a one-off.
Some are just images, but others have button layouts on them to show what button to hold for which payload. You just have to name the payload files correctly to get them bound to the correct key. I believe the Luma installation instructions cover it in more detail