The easiest way to do anything to patterns is to copy them onto craft paper, or use paper grocery bags taped together. First trace the size you are using then remove the seam allowance, or at least draw the seam allowance on the pattern so you can get a measurement of what it will be when you sew it together. Then you can easily match your patterns to make sure they will fit each other....and you. Remember if you have a body measurement you have to add "ease" to it so it the fabric will fit around your body. For example you need at least 3-5" ease around your hips because when you sit down your hips are bigger than when you stand up. So, if your hips measure 38" around the fullest part of your butt then you'll need the hip measurement of your pattern to be about 41-42". Of course, if you are working with stretch material the ease is different. You don't need much, if any, ease in a pair of tights or bodysuit made of stretchy fabric because it will stretch with you when you move.
It's always a good idea to go by the measurements given on the pattern instead of the size, especially if you are using vintage patterns because sizes are all different. What was a size 12 in the 1950s is now about a size 4. Always go by what the measurements say and always use your biggest measurement of hips, bust, waist to choose the right size pattern. You can always make the other parts smaller in the fitting stage.
If it's a really important project be prepared to make a mock up of less expensive fabric so you can get the fit right before cutting your final fabric. Good Luck
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