I have taught myself to draft and modify block patterns using a book called 'Pattern Cutting and Making Up The Professional Approach Volume One' by Martin M Schobren & Janet P Ward. I have tried using a few different books but this one is by far my favourite. It shows you how to take measurements and draft a block pattern to your own measurements. It also teaches you to customise your block to create pretty much whatever you want. It doesn't have the easiest to understand layout but I have found that the content is excellent. I drafted my first trouser block using this book yesterday and it fitted me perfectly!