Without seeing the whole pattern, I would guess that you sew the pieces of interfacing together just like you did with the pieces of fashion fabric. Then place the fashion fabric and the interfacing right sides together (in this case the interfacing does have a right and wrong side...the wrong side is the side with the seam allowances sticking out). Sew all the way around the outside edges, leaving a small pocket to turn the whole thing right side out. Then you can topstitch all the way around it if you want.
I made a dress like this last year. Different pattern, but nearly identical dress. I used the Sew Baby One Piece Reversible Sundress pattern. It uses one big piece for each side instead of three pieces sewn together. Here's a picture. You can see how you can kind of see the reverse side fabric (stripes). It's even more visible when it's being worn.
To answer your question, if you decide to make it reversible you can either piece it together like you did with the first fabric, or you can cut it out as one big piece. Whichever is easier.