What kind of chip do you have? What version of the bios do you want to run .06 or .67? give more info next time.
Anyway, since I just finished doing this and I know its hard to find the right info I'll help out. This info pertains to flashing a 2.X pro chip.
1. get the X2bios manager from here
and follow this tut to the T http://www.teamxecuter.com/biosimage.htm
You have succesfully created your 512k bios!
2. FTP flashx to your f:/apps folder AND ftp your newly created 4981.?? bios to a folder called c:/bios (if it doesn't exist then make it). Note: flashx doesn't like IGR and TSR enabled (at least mine didn't) so disable TSR in the evox menu and edit your evox.ini to disable IGR if flashx freezes on boot screen. Once bios and flashx are FTP'd power down.
3. Put your chip in flash mode (dip switch's 123 off and 4 on) and power on.
4. Head over to Apps and run flashx. choose option to 'flash bios' and choose c:/bios as your locations to flash from. You should see the 4981.?? 512k bios you created.
5. Choose the bios you created and it'll ask which banks you want to flash to, 1 &2 or 3& 4. This part is up to you. What I did was flash 4981.06 to banks 1 & 2 and then flashed Cromwell 2.28 to 3 & 4 in case I ever need to recover my box. For purpose of finishing this I'll say choose 1 & 2.
6. Once you choose which set of banks to flash to hit the correct button that the onscreen instructions tell you to (Y I think?) and it'll ask you to confirm choice so hit the correct button again.
7. You'll first see the old bios erased and then the new one put on, it takes about 20 seconds for all this to happen. Now power down and put your dip switch's in the correct position so its boots from banks 1 & 2 (see the layout here
You're done! you should see the new bios reflected when you look in evox's settings. Remember this is only for flashing a Pro chip, not a lite.
This is every step from start to finish and if you still dont get it then step away from the computer, forever.
I cant believe I typed that all out. I hope it helps someone cause I sure could've used it about a week ago.