You should now have 5 parts sewn together.
With rights together , sew interface across the top of all 5 parts. Start at the middle and sew to the end, the interfacing , the facing will stick out on both sides . ( This is done for new sewers who find it hard to line up) ..
Flip interfacing to the wrong (back side) of the dress.
It's a good idea to iron or stictch it in place