I'm no expert on this but I do know that getting the bottom level, even, and the right depth is important. Two tricks I've seen is using wet sand (or dust) and using 2' x 2' pieces of 1/2" plywood to step on when doing the final leveling and smoothing, and never putting a foot on the bottom. Use the plywood when standing on the liner as well to protect the base.
The shop-vac is necessary.
Other than that, others will have to help you.
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