I'm only going to describe slot2 because it's the only type that can support GBA games.
You have to buy the following items:
- Supercard Lite (DO NOT GET RUMBLE DO NOT GET RUMBLE NO RUMBLE NO NO NO, RUMBLE DOES NOT SUPPORT GBA, DO NOT GET RUMBLE)
- Superkey (This device is needed to fool the DS that you can play DS games)
- MicroSD memory stick, Micro is the tiny one, you'll probably want an adaptor to use a regular card reader

(I bought something like this
http://www.amazon.com/SanDisk-MicroSD-A ... 55-1124431 )
- A USB card reader to transfer your patched games onto your memory stick
So, basically, once you have all that:
Download the supercard software from their website, there's 2 things you'll need:
1) The update file to update your firmware
2) The program that runs on your computer that patches NDS and GBA games into the format that the supercard reads. (This is really simple to use, if you
really need a guide for it, ffs just search for it. fifthe1ement or w/e his name is should have a good guide)
Now, after you copy all your files into your memory stick (ALL files, each NDS game has 2 files, GBA has 3 files) put it inside your supercard and put it on your GBA slot and run.
To execute the firmware updater, just run the update file (make sure you also upload that, don't need to patch it lol)
Now, your DS can play any GBA game.
Saving using GBA mode:
When you use an ingame save, after you save, hold down L R Select A, this brings up a menu asking if you want to save. Click yes and it updates your save file. You have to do this every time you save if you want it to actually save. It's not that bad, really, it becomes a habit after a while.
GBA mode also has another feature: Real Time Save. Think of it as save states that you have in emulators, to pull it up hold down L R Select B, and you'll get a save/load menu, choose what you want. (Note: For some games, mainly FF games (all i've played lol) you might get a black/blue/whatever screen when you load, just hit start to go to menu and back, or save your state inside a menu to avoid it alltogether. Don't save while talking or in a battle, or it might screw up lol)
Now, to play NDS games, you need that Superkey, which you put into your NDS slot. This device tricks your DS into thinking that it has a DS game in it, and so it lets you play DS games.
If that's placed inside, you can run both NDS and GBA games on your supercard. DS games save normally, so you don't need to hit commands to save. (Also note that DS games don't have the real time save feature)
That's about it.. I think? Might have missed something
edit: oh, and yes, the Supercard Lite fits flush and doesn't stick out at all.