To edit Grub2 during the boot process try the following:
Immediately after the BIOS splash screen during boot, press and hold the SHIFT button. This will display you grub containing a list of kernels and recovery options
Press e to edit the first kernel displayed
Find the line ending with quiet splash. Add your boot option before these key words - i.e. so the line looks like [...]nomodeset quiet splash
Press CTRL + X to boot
Follow the steps in Coldfish's answer on how to fix the nomodeset boot option permanently so that you don't have to go through this manual procedure again.
Best Answer
You put
nomodeset
between thequiet
andsplash
part.