Hey assuming you have already modded said xbox, follow these directions:
I had a similar request about a month ago and posted here:
http://coinopsproject.freeforums.org/viewtopic.php?f=43&t=660If you want the purest vga source do this:
1. grab a vga cable - cut one end off and split the wires out.
2. grab a stock xbox av cable and remove the cover on the end and sheer off the cable part --chuck the cable but you need the connector part that plugs into the xbox.
3.Solder together following this guide:

4. Check
http://ps-2.kev009.com/sog/ for if your monitor you plan to use can utilize Sync-On-Green (SOG) --most monitors after 2002 can i am told
5. If it can SOG than you are in luck and can try without building the circuit first and using the vga bios dvd bootable to test.
I cannot link the burnable DVD here obviously i'm assuming it uses microshaft code, but it can be found online and if you cannot find it pm me.
If your monitor doesn't support SOG, I have purchased 5 LM1881's and can send you 1 all you need to do is send me shipping cost
Good luck.