I won't flame you for not searching the forum....
Here you go. Here is a good starting point.
www.OmniaRom.com has some good Omnia i900 and i910 flashing info. There are a lot of cross posters between PPCGeeks and OmniaRom. Here is a good how-to-flash thread. I just picked this one because it's the first one I found doing a quick search:
Flashing How To:
http://omniarom.com/forum/index.php?topic=486.0
UMDL - notice the link to the omnia_umdl.rar file at the bottom of the post in the link above. You need this. It does the transferring of the new ROM to your phone.
ROMs:
http://www.omniarom.com/forum/ - see the Featured Chefs section.
http://forum.ppcgeeks.com/forumdisplay.php?f=83
Windows 7:
You can use Windows 7 to flash your ROM. You must right click on the UMDL.exe file and select compatibility mode and select Windows Vista, and check Run as Administrator. From that point on, it will work in Windows 7. I just did this a week ago without problems.
A word of advice. Be absolutely sure that you understand the steps involved before you try to flash. If not done correctly, it's pretty easy to toast your phone, or 'brick' it. There is a step involved that requires you to unselect 'allow USB connections' inside the Windows 7 Mobile Device Center (called ActiveSync on WindowsXP and in the instructions in the link above). You must make sure you do this or you won't get it to work.
Good Luck.