Adding a return to an AG pool.

[haze_1956]

How I would do it -

Unhook the liner and put a couple layers of duct tape on the inside of the wall where you plan to drill through.

Then put a large sponge or some soft foam and a piece of wood between that area and the liner.

Then using a hole saw, drill carefully until you are through the wall. The sponge pushing outward should seal the opening and stop any debris from falling inside the wall. The wood should protect the liner in case the pilot bit on the hole saw goes too deep.

Diagram for reference