Copying the fit of a shirt I love — how do you take it apart on paper without cutting it up?

The shoulder is usually not width, it is slope. Two shirts with identical shoulder seam lengths fit completely differently depending on the angle of that seam, and a slope that matches you is what makes a shirt sit without a diagonal wrinkle at the armhole. Lay both flat with the fronts together and look at the angle rather than the length; the difference is often small enough that you will not spot it with a tape measure.

If you do want the actual shapes, pin-tracing works without any damage. Lay one section flat over pattern paper, pin through the seam lines every couple of centimetres, lift the garment off and join the pin holes, then add seam allowance. It works well on the body pieces and it is fiddly and unreliable on a sleeve cap and a collar, so I trace the flat pieces and take the curved ones from measurements and comparison instead.

Start with a flat measurement chart rather than a rubbing, because you probably do not need the whole pattern. Measure it laid flat and doubled: chest at the underarm, waist, hem, shoulder point to shoulder point, back width halfway down the armhole, armhole depth, sleeve length and bicep. Write all of it down, then write the same numbers from the finished measurements printed on your pattern. The two or three lines where they differ by a real amount are the entire reason it fits and everything else is noise.

Photograph it flat on a cutting mat, straight down from above rather than at an angle, and keep the picture. The grid gives you a scale reference and you can measure things off the photo later that you forgot to measure at the table, which happens every time.