When I was sewing clothes as a teen, I didn’t do any prep work except prewashing the fabric. I cut out my pattern in my size and away I went, cutting and then sewing whatever it was I was making. Boy, were those the days!
Nowadays, I never know if a pattern will fit, even if “my size”. Maybe it will fit in certain areas and not in others. What’s a seamstress to do? The answer is usually “make a muslin”.
A muslin is a practice run, or a sample of the garment in the size(s) you think you’ll need. You don’t always need to sew the entire garment, perhaps just part of it. It depends on the garment and how much ease is involved.
I’m working on a jacket project and I was uncertain about the sizing. Time to cut up and sew a muslin. But first, I graded up the pattern one size, tracing it onto medical exam table paper I bought online.
I couldn’t find any muslin (why muslin? It’s relatively inexpensive woven fabric, perfect for sampling). What to do? Oh yes, I remembered saving the exterior shower curtain I removed from the bathroom in this house when we moved in. I didn’t care for it, so removed it, washed it, and placed it in my “you never know what I may need this for” pile.
Today it became my muslin. Gary was incredulous, “you made a jacket out of a shower curtain?” I must admit it does sound rather funny.
I didn’t finish the entire jacket, I merely needed to check shoulder fit, sleeve fit, and overall length. And, as it turns out, the grading was almost too much. I will trim that back in the first actual jacket I make.
And a bonus: now that I have all my pieces drafted out, I can plot them on graph paper for a more narrow piece of fabric (handwoven!). That will be finished jacket #2, I hope!
Magpie Dye Studio 
Shower curtain jacket!! At least it would be showerproof!