Ensure the surface has no paint or oil etc.
Rough up the area with course sandpaper.
Wipe the area with a clean rag lightly dampened with some Acetone. Be very careful with Acetone it will destroy paint surface and read the H&S data. You can use the Acetone to clean your brushes and mixing container afterwards.
Cut 2 or 3 ?Patches? of fibreglass chopped strand mat. The patches need to be progressively smaller so the second patch applied is smaller than the first.
Then apply the mixed resin to the car surface and fibreglass patches and position the first patch.
Stipple the patch with the brush adding more resin in needed to soak the patch. Do the same with the other patch/s.
Note: - If doing the upper surface, use less patches of chopped strand and finish with 1 or 2 patches of fibreglass Tissue. This leaves an improved surface.
And as noted above by Bigbaldbloke, cover up everything with plastic sheeting and stipple if necessary until resin is starting to go off.
I purchased my products from East Coast Fibre Glass supplies. They also have some video guides.
I have no association, just a satisfied customer.
https://www.ecfibreglasssupplies.co.uk/