ok, right click on the rom u want then go to edit cheats and pick out the 1 u want then checkmark it
to add a cheat right click on the cheat select board and click add new cheat
type in the name
then the 8 first numbers then the 4 last numbers
it should look something like this
_example_ 12345678 1234
for the GS button to work in the game hit F9.
the rest is in the project64 user manual/cheats help
which were you should have looked first