How to make frustum of a truncated pyramid solid

I am pretty new to Sketchup and I don’t know all the right words. I’m trying to make the walls of a truncated pyramid solid. I don’t have problems with straight walls, but I can’t manage with sloping walls…

The link to the file is here, so you’ll see what I mean.

Will this do what you want? Each face must be flat so you need to draw the outline of each face so they lie on the same plane. I added the edges shown selected and repeated that on the other side after deleting the edges that didn’t work at the deep end of the pool.

Thank you very much! That helped.

Group the parts of the pool together into a component. Make it easier to work with–separate from the ground surface, when you use View / component edit / hide rest of model. Grouping the other parts of the model into components would also allow you to put them into layers and turn off visibility while you work on the pool etc.