Learning to make Muslin Patterns
While it is simple to make a muslin pattern from a ready-made sewing pattern, it is more complicated to learn how to then properly alter the pattern to fit you. Some pattern companies offer special fitting patterns which allow you to make a muslin and then show you how to make specific alterations to it. These fitting patterns can be very useful, especially for the beginning sewer, but they are not ideal since they do not show you how to make alterations to other styles of garments. If you want to really hone your fitting skills, checkout some of these helpful books on patternmaking.
How to Use, Adapt, and Design Sewing Patterns: From store-bought patterns to drafting your own: a complete guide to fashion sewing with confidence by Lee Hollahan
Make Your Own Patterns: An Easy Step-by-Step Guide to Making Over 60 Dressmaking Patterns by Rene Bergh
Basic Patternmaking in Fashion by Lucia Mors