I'm a DIYer finishing my basement. The floor is concrete, which I will tile. The builder left a rough-in for a tub and toilet. I've framed the area for the tub and plan to install a one piece acrylic tub/shower combo, Delta model 226032A. I get how to do all the plumbing for the supply/mixer/showerhead. I understand how to connect the drain assembly to the tub. I'm lost on how to attach the assembly to the rough-in stub that the builder left. I assume that I put in a trap, with the opening lined up with the straight pipe coming off the drain assembly, so that the drain slides into the trap. But how am I supposed to attach those pipes? The trap is down inside the concrete, and it will all be covered by the tub once I put the thing together. I'm confused.