Copying a garment
I've copied two garments going straight from the garment to a pattern. I've also used a worn purse to copy the shape of the inside to sew a fresh lining. This didn't make the purse new, but it definitely made it more workable. This is sewing related, but you could apply the rules to knitting with some caveats.
Check the shape your source is in. A worn out and ragged pair of shorts could be ripped into pattern shapes and copied. A worn out sweater is likely larger / smaller than the original copy of the pattern. Anywhere with a lot of wear / holes might show that the pattern is a little small on the spot, and might distort the original pattern shape. If you are not tearing apart the garment, you can copy the shapes on to paper / using pins to mark the pattern shapes. The more fragile the garment, the harder it is to manhandle it into copying the shapes and the less accurate your copy will be.
Try the garment on, if you can. Note anything you don't like. If the shoulders are too long for your shoulder, note the approximate amount. Check if the sleeves are the right length for your arms. An accurate copy will not lose these problems, so knowing that you might want to tweak that is important. An inaccurate copy may make these problems worse.
Find some fabric that is similar to the original fabric. A slight amount of stretch will simply make the test garment "roomier" as it shifts around you. A major difference in drape / stretch will result in a different garment. If you want to demonstrate this, try on a knit garment and a stiff canvas / denim version of the same garment. If the shapes are the same, it will hang differently on you.
Walk the seams. Using a piece of paper or a measuring tape, go around the curves / edges of a garment. I prefer to do this prior to adding seam allowances, since you're talking about the finished garment shapes. Shoulder seams / side seams will often be different. A difference of 1/2" in a side seam can often get lost in the hem, and a bias seam may lose or gain the length as the fabric settles. A shoulder seam ideally should be a similar size, especially if you need to set a sleeve in it.
You are starting, I assume, with something that fits you. However, the copied pattern shapes aren't guaranteed to fit you. Take measurements of your body. For a top, I would do biceps, sleeve length, widest point of the hand (if not doing a cuff,) neck, shoulder, chest at armpits / bust point, chest at belly, hip if you're going to go down that far, armpit to hip / where you want the shirt to end. Double check the pattern will fit those numbers. It's easy on a shirt, for example, to lose width when you've got a 3D curved shape over the body, and you're trying to copy it flat. If your shirt just fits over a wide part of your body, you may want to add a bit more to your test one, since a fresh fabric will be less stretched in those areas.
Copying a collar stand is unpleasant. You can find people on the internet who talk through how to draft a collar / collar stand. On average, I'd make the collar stand a little long for my estimated curve, since you can trim it to be smaller.
Add seam allowances to the paper pattern and hem allowances if you wish. If you do not, carefully note that you did not add allowances for hems / seam allowances.
Sew the garment with as much care as you have the patience to use. If seam finishing will use more fabric, you should use it in your test. Ideally, try wearing the test copy for a while. Many fabrics change how they sit on you after they're warm and worn for a while (think of a pair of jeans fresh from the dryer or a pair after several wears.) Pay attention to irritation (do you need to bind a seam / serge a seam to have less seam allowance in an area?) or fit issues (do you need to add / remove fabric?) Is a seam under a lot of strain? Does that match the original garment or is it something in the drafting? If the result is so alien to confuse you, can you buy a pattern of a similar garment or watch someone sew a version of it to see the difference between your pattern shapes and the pattern?
Redraw the pattern, noting your changes, and try again. Repeat until you're happy, or you've got enough workable test garments to be done.
