If you end up in a situation where you have a finished garment sample but no pattern, you have three options. (Because if you want to go into production with this style, you’ll need a pattern, not just a sample.)
You can take the sample apart and trace or digitize the cut pieces.
This is the most sure way to get a pattern from a garment, but it is not perfect. That particular sample might be slightly off spec or it can be difficult to fully remove all the seams without compromising the fabric. Even if you iron the pieces and lay them down carefully, it is easy for the grain – and therefore traced pattern piece – to end up skewed. Small differences in edges or seam allowances can be hard to tell from the cut pieces, but make all the difference in a good pattern.
You can use the sample as a reference for each point of measure as you draft a pattern.
This is a good option if you don’t want to take apart the sample. If you have a good pattern block to start from, this method becomes even easier. This option essentially uses the sample as a really accurate spec to create the pattern from.
You can draft a pattern from scratch using the sample as inspiration.
If you don’t have access to the same to take full measurements, or if there are aspects you want to change, you can use the sample simply as a visual reference the same as you would a inspiration photo. With this option, the main goal is to note the silhouette, style lines, and proportions of the sample, but create the pattern from scratch.