Firstly, are you on about the grille or bumper? I thought bumper seeing as thread title states bumper. But technique is same
BUY
Halfords - ready mixed fibreglass (not as messy as other fibreglass)
Halfords - Easysand filler (sands smooth as a babies bum)
Halfords - sand paper and wet and dry paper
DO
Cut slates out and sand down until no bits stick out
Fibre glass, do both sides to make sure strong but keep thin to start as it can be a hassle to sand
Let dry
Give another thin layer and sand down again
Let this dry
Layer on filler
Sand down
When you have the general shape and you're happy
Wet it and sand with wet and dry paper, this'll make really smooth
Basically, make sure the holes are filled and smooth away
Hope this helps