It was a long time ago that I done mine so sorry if my instructions are a bit sh*t.... from what I can remember I used samread/samwrite with extreme 5.2.
Power the PC up(booted onto a DOS bootdisk) with the XBOX on and the SATA cable connected.
Power the 360 off/then as you power it on run samread. I think it depends on what program you are using but you should then see the SATA chipset in the list....
Then Power the 360 off select the correct chipset option...... wait for about 10-15 seconds then power the 360 back on.....then if all is well it will start to read the firmware.......well thats the idea anyway. Then once you have created the new firmware do it again but this time with samwrite hope that helps.
REMEMBER DO A READ FIRST AND BACKUP YOUR ORIGINAL FIRMWARE OVER AND OVER AND OVER and over just to be safe
oh and another thing I'm guessing that you are aware of the current issues with using a modified firmware on XBOX live ??? If not read the front page of the site.
James