Mark the position of the legs on the underside of the seat. The back legs should be as close as possible to the back, without cutting into the slots for the back slats. Use a ruler to make sure the legs are lined up square.
Use your mini drill to make a hole the correct size for the peg you created on the top of the legs. The front holes should be drilled at a slight angle to make the front legs angle forward as shown in the photo.
Test fit your legs in the holes.