Lay your front body right side up, next to your back body piece, also right side up. The sleeve will want to get in the way, but just fold it out of the way.
Then, flip your right body on top of your back body so they are now right sides together and pin the side seam, paying attention to match the side seam notch for the waist.
Starting from the under arm, stitch the side seam closed down to the hem with a 1cm seam. Overlock the side seam and press towards the back body.
It is probably easier to do the next step (the armhole and shoulder seam) on this side first, before moving on to sew the other side seam - as there is a lot of fabric that will get in the way. Once you have finished the shoulder and armhole, come back and do the remaining side seam.