(1) Sew up the back seam
You want to make the back two pieces into one back piece, so sew up the center back seam. The outside fabric should be facing, and you should have a fairly thick fabric sandwich - lining, fabric, fabric, lining.
|Sew the center back seam first, so you have a front and a back.|
(2) Sew the crotch seam
Very straightforward, and easier to do while everything is still mostly flat. Again, make sure the outsides are facing each other and that you have the fashion fabric between your lining fabrics.
|Sew the crotch seam together.|
(3) Sew the side seams
At this point you should already have the pieces in the correct position, so you just have to stitch down the side seams. It might be more difficult if you have very curvy seams from using multiple sizes (like I have to use), but just go slow and it should be fine. Don't rush or pull the fabric through the machine - let it feed smoothly on its own. Just guide the fabric with your hands. If you try to force it you will have a puckery mess. This will take some practice, especially with stretch material.
|Most of a leotard.|
(4) Sew the shoulder seams or fabric straps
I would recommend putting on the leotard and marking where you need to sew the straps, otherwise you may have to adjust later. Easier to measure first. If you are sewing fabric straps (like my red/purple dress) sew the sides, then try on to mark where they need to be sewn over the shoulders. If you are using elastic straps (I typically use 1" elastic, wrap fabric around it and sew to make them) then you should wait and attach these after the elastic is inserted into the leotard.
|Straps pinned and ready for sewing.|
(5) Check fit and make adjustments
It is important to check the fit before you add elastic. Is it gaping? Is swayback your nemesis? Is it droopy in the front? Too big over the hips? I either take in the side seams, or I add darts to the front (under the bust to about the belly button level) or back (pinch out gapping excess and mark dart positions with pins) until the body suit is quite well fitted to the individual.
|Need to pinch in the side seam.|
And that's it for today! At this point you should have an elastic-less body suit, which we will finish with elastic and (if necessary) bras next time!