I always put in the overflow, no gasket then the drain no sealer or gasket then dry fit a pipe into the overflow and hold up the T mark the pipe cut it then do the drain hold the T off to the side to mark drain pipe, dry fit before gluing sand if necessay to make sure pipes are seated. A short level helps when marking the pipes, make them plumb and a little slope on the drain. Be sure to make perfect straight cuts, a power miter saw is great. PVC saw works, hack saws suck. Use primer and glue on the plastic if it is ABS or PVC. You will hear you don't have to, don't listen, primer and glue both.
I always use 100% silicone for sealer on tub drains, never liked plumbers putty. I put a little on the over flow gasket too just to keep it from leaking. Do not use anything with latex in it including tub caulk, 100% silicone, get it wiped off quickly, blue shop towels to start and a nice soft cloth to finish.
Also you need a good helper and a plumber's dumbell, tub drain wrench or strainer wrench, to install the drain. Two screw drivers work, but a pain.
Start the strainer into the plastic by hand, gently let the threads find each other, this is where the good helper comes in. I get the drain in the tub, then glue the pipe back then the T then the up pipe then get the overflow and gasket. Keep the blue shop towels handy and wipe off excess glue, helps look like a pro did it.