Work fast... you need a little time left before it starts to harden.
Lay in the cloth/mat to build up as needed. (wet with resin)
Cover with a peel ply (some of this come in the bondo fiberglass kit for car bodywork.. or any decent fiberglass supplier) working from the bottom up, to not have any air bubbles.
Cover that with a board (for flat) or coroplast (for curved) to hold the peel ply firmly in place and give a smooth surface. Leaving some at the top you can pull back a bit.
Pull that top back and drip in about another ounce or two of resin per square foot of patch. This will soak down and push against that peel ply.
Get it right and you have very little sanding to prep for paint/gel-coat.
Because of using the peel-ply, you don't need surfacing wax in the poly resin. This makes the gel-coat/paint and any future repairs stick better.