You can build Decrypt9 by using ./build.sh in the root folder. You need DevkitARM to build.
Binaries can then be found in the build/source folder.
You can use http://dukesrg.no-ip.org/3ds/go to load the Launcher.dat on your 3DS. This should work on almost any firmware version below 9.3.
Then use ncchinfo_gen on your encrypted game (dump the game with the Gateway launcher).
Then, if you're on firmware that is less than 7.x, create/edit slot0x25KeyX.bin in a hex editor and put in the 7.x KeyX (no, I won't give it to you).
Place ncchinfo.bin (and slot0x25KeyX.bin, if on less then 7.x) on your SD card, and run the decryptor. It should take a while, especially if you're decrypting a larger game.
Roxas75 for the method of ARM9 code injection
Cha(N), Kane49, and all other FatFS contributors for FatFS
Normmatt for sdmc.s as well as project infrastructure (Makefile, linker setup, etc)
Relys, sbJFn5r for the decryptor