When making a practice garment it is best to use a fabric as similar to the fabric you will be using.
I have a generous stash of poor quality acrylics that look like woolens for sampling jacket patterns.
Muslin is often used for shirts and dresses, but you need something a bit heavier for trousers, jackets and coats.
Old sheets also work well for dresses and shirts.
If you have several alterations to do, I would highly recommend buying at least one book on the subject so that you will have a starting point. Then utilize help from the forum to help fine tune the adjustments.