Typically you make a wooden mold out of plywood and construct the fiberglass on the inside (just like a boat)-- You add the fabric and build layers. Always do the inside so you can shape easily. Wooden shell can be stablized with 2x4's for strenght . Other ways are you coat the galvanized wire with the fabric and resin . The wire is an internal support (like wire in a concrete slab or wall). It can be 90 degrees like a tank or you can slope to make a sloped pool. .

I would add a pipe for a drain ( there are what they call bung fittings that you can use.

This is real easy to water proof -- you just need to work with the fiberglass cloth and resin. Its messy to work with . Easy to fix when you screw up -- unlike concrete or carpentry.

Best regards

Ed