I have used
this site to figure this out but I am going to explain it my way, because they put in un-needed downloads.
Things you will need--- 1. Xbox controller cable 
2. Female usb cord 
3. Flashdrive 
4. Wirecutters/Scissors 5. Electrical tape 6. Xplorer360 ---- DOWNLOAD HERE! ---- (Optional) - A soldering gun if you wish to solder it and make it more reliable.
I am not 100% sure about these but here is a list of devices that do and dont work on the xbox as a memory card.
Xbox USB Compatibility List - Xbox-Linux Step-1. Cut the female usb cord leaving space for it to be stripped and extra room. Cut the xbox cord from the controller again enough room for it to be stripped and have extra room. Step-2. Strip both the female usb cord and the xbox controller enough so they can be soldered together or twisted. Pull away any foil sheilding, exposing wires. Step-3. There will be A Red, Black, Green, and White wire on the female usb cord, also on the xbox cord will be a Yellow wire just cut that off or just leave it there, it wont be used. Twist or solder the RED to RED, BLACK to BLACK, GREEN to GREEN and WHITE to WHITE wires together. Step-4. Cover all the wires together so that none of them touch with the electrical tape. Your cable should be something like this. Without the flashdrive in it and not cut off. 
Step-5. Insert your flashdrive that you are going to be using into your computer. (NOTE)-Everything is going to be deleted on you flashdrive so please back everything on it up on your pc. GO into MY COMPUTER and right click on the flashdrive, make sure it is the right one! Then right click on it and choose format. Format it to FAT32. Step-6. Take your newly made cord and formatted flashdrive to your xbox, insert your newly made cord into a controller port and insert the flashdrive into the other end. Go to Memory Mangement and there should be a controller port, click on it and it should say that it has been erased. Your flashdrive is now formatted for the XBOX! DONE!!! You have successfully made an action replay device with the amount of memory as pleased thanks to your flashdrive. Use Xplorer360 to drag files onto your flashdrive when inserted into your pc by clicking DRIVE \ OPEN \ HARDDRIVE OR MEMORYCARD. Then click on PORTION 0. Thanks to Hackosis - abnormal condition or process of hack and Shane ME! UPDATE
I now put the controller back on the cord and put the flashdrive slot into thefront memory card slot and soldered it inside. I may add a tutorial for this, but it requires soldering and I will only possibly add it by request. Heres some pics.
