Ill make a quick Basic TUT
on your SD card make 3 folders i use.
GBA_SIGN - This folder is where the 3in1 will store game data
GBA_SAVE - This folder is where the 3in1 will store your game saves
GBA Roms - This is where you put your GBA games
I have
GBA Exploader into the root of my SD for convinience
ok now you have ur games ready and your gba_exploader, run gba_exploader from your DS
1. you should have a blue color GUI and be able to search your folders to find your GBA roms now
2. select your game and press A to run (all the options are given to you on the bottom screen)
3. if you want to use your 3in1 cart to act as a normal GBA cartridge press the R button on your DS this will switch your screen from blue (PSRAM mode) to green (NOR mode) and L to go back to PSRAM mode.
so you have 2 basic ram modes
PSRAM will write to the 3in1 cart faster but will lose your rom when the DS powers off (wont lose your saves though)
NOR will write a bit slower but after powering off your DS the game will stay loaded acting as a normal GBA cart
Tried to keep this simple hope this helps