When I worked in industry we would buy certain key garments and take patterns from them. The silhouettes could then be manipulated into various styles of garment! I use this way of working even now as I still cut patterns from favourite items of clothing.However copying garments is not for the faint-hearted and it can take hours of tracing, measuring and remeasuring.
The best way to do it would be to take the garment apart, bit by bit, press and draw round all the pieces, this gives you all of the pattern pieces. Remember the seam allowances are already there so mark them, mark the grain lines and note on each pattern piece how many you need to cut out. As you take it apart, note down the order of deconstruction, reverse this and you have the correct order of construction. So you have your complete pattern, with the relevant information written on and a list of how to make it up. However no one usually wants to destroy or re-make a bought garment in this way and if you want to copy the pattern to make another garment, then it is often a favourite item of clothing.
The art of copying favourite garments. Start with something simple. I want to copy the shape of my favourite pairs of jeans. I want to copy them because they fit me so well. They are in black denim and from the Everyday Skinny petite range from Next. I have bought other colours from the same range, in the same size and style but none fit me as well as the black pair. When I take the pattern from these jeans I will make them into a pair of trousers for work.
This is the finished pattern. Add seam allowances. I usually use 1cm seam allowance for attaching waistbands and 1.3cm for the other seams. Add fly shaping to the centre front. Add the grain lines and any other information you might need, I usually write on what garment I have taken the pattern from.
I made this pattern up really quickly in a 100% cotton trouser taken from my recycling bag.