I am a boatbuilder & used to do big GRP repairs for a living. First its a pond so it hasnt got to look marvelous. Grp doesnt have to be done from both sides it can be done from either side.
What you do need is to sand or grind the surface back to clean material preferably to a feather edge in the middle where the crack or hole is. I use fibre backed sanding discs on a grinder for this, its gotta be clean. A wipe over with acetone is not a bad idea but not essential if its been sanded back well. In weather like this you are on a hiding to nothing unless you can heat it, Infra red heat lamps are good for this, In fact i have done grp work when it was snowing using one! You should be using Chopped strand mat, for fixing a hole in a pond like that two or three layers of 450gm CSM will be more than adequate.
Do not exceed the max catalyst ratio it wont speed up the cure if its cold.
Cut your glass into patches the right size or on big runs have a stack of oblong pieces ready cut.
Applying the resin. You will hear much about Stippling the resin into the mat, Chopped strand mat has a binder in it which holds it together while the resin soaks in. Waste time stippling & it will fall to bits when you go to put it where its to go!
Nobody who uses glass for a living does this anymore. I use a wetting out board, bit of plywood, cardboard, take your CSM piece one at a time slap resin on it, turn it over & do it the other side. Put it where it is to go then do the next bit the same.
Trick to good fibreglassing is to get the air out & consolidate the laminate. For this you need a paddlewheel roller for size you need 1 1/2" to 2" long will be fine, when the wet glass is in place roll it down & the roller wil get the air out & hey presto you have done it!