I didnt use glue as well mainly because if it stretches i can remove the cover & refit easily.
Just simply fit cover to seat then starting at 12 oclock stretch cover & staple, then 6 oclock, then 3 oclock, then 9 oclock. Then work your way around the 4 sections, stretching, folding & stapling the seat cover doing opposite sections.
If you are using a manual staple gun you might find it easier to warm the plastic seat base first so that the staples go all the way home. If you have a powerful air powered staple gun then they will just bang straight in no problem at all.
Take your time & gradually work you way around the cover stretching & stapling. If you make a mistake, prize out the staples & do it again.
Im sure there are several "how to" tutorials on you tube.
Good luck!
