I'm new and need help with my first hackintosh.

I'm trying to install it on a acer 2420, old machine. Have follow this guide and finished installed except that after i'm booting up into a blue screen.

I know there's this step to try but as i'm new to this, I don't know how to revert to AppleIntelGMA* and install a keel

Can someone guide me allow? Thanks a lot