If you look at a seam - whether serged or sewn - it occupies a strip of fabric along the edge of the pattern piece, and even a very narrow stitch will need fabric on both sides so it has something to hold on to. That strip is no longer part of the surface, thus a seam effectively trims an edge off the cut fabric pieces.
For instance, my serger's stitch is 5mm wide, so a serged seam joining two pieces removes a total of 10mm of fabric, and on the raglan sleeve, there are six or seven seams around the neckline. That's 60-70mm used up in seams, which is a good third of my neck diameter; if I didn't do something to compensate, I'd throttle myself.
The remedy is simple: since a seam effectively trims a little off both of the the edges of the drafted pattern pieces, add on enough to counteract the 'trimming'.
For the body, legs, and sleeves of the catsuit, you can ignore seam allowance because the fabric is stretchy, and because the size of the seam allowance compared to the piece of fabric is insignificant.
Where the amount of fabric taken into seams is significant compared to the pattern pieces, you must add seam allowance.
Conventionally, a fairly substantial seam allowance is added - 5/8" - and most sewing machines have marks around the sewing area to help the user guide the fabric using the seam allowance cut edge. I personally prefer only to add as much as my serger would normally trim to - 5mm - and pin and feed the fabric carefully. With a normal machine, I generally use a conventional seam allowance, because a conventional machine's feed works better with one.
To add seam allowance, just measure perpendicular to the simple piece on the pattern. You can either sticky-tape extra tissue paper to the pieces, or copy the pieces onto new paper.
Where you have an internal corner (see diagram), you should snip through the seam allowance at the corner, just shy of the drafted corner.
Where you have an external corner (all the other corners in the diagram) you should snip across the corner. That can either be just shy of the actual drafted corner, or from corner to corner of the added seam allowance.