You got a second? Then pull up a chair and let me tell you about the best patterns I’ve found in a while for modifying costumes, Jalie Patterns!!

Jalie is a French-Canadian Pattern company that specializes in patterns for sportswear, swim wear, and anything stretchy. From their site:
“All our patterns are fitted on real people, not mannequins and sew all our prototypes with a home sewing machine, not industrial. We want to make sure that you will be able to recreate everything we make without breaking needles. We sew and sew again until we find the easiest way possible for you to achieve a professional result.”
Their patterns come in not five, not 10, but twenty-two sizes in just one pack! To top it all off, they have a Youtube Channel for all sort of tips and tricks, including walkthroughs of many of their patterns! https://www.youtube.com/user/jaliepatterns
However, there is one big change from your usual Simplicity or Mccall Pattern. These patterns are meant to be kept, and are printed on heavy paper not suitable for fabric. That’s where this stuff comes in!

This stuff is Swedish Tracing Paper. You can mark it up all you want, and it’s durable enough to draw on without snagging, but lightweight enough to drape and act like fabric! You can trace your pattern and then stitch this stuff together instead of muslim! Then rip your seams and have your pattern ready to go! You can find at Birch Street Clothing’s Site. or over at amazon.
You can find the patterns for around $13 on their site, http://www.jalie.com/. Amazon.com also carries some of their patterns as well.

One pattern I highly recommend getting is their “Jalie Skinsuits Wetsuit Swimwear Women Girls Sewing Pattern #3135”. In all my years of sewing, I’ve never found a better fitting, more customization pattern for a catsuit/jumpsuit than this. This has been my go-to sloper since I got it for all things superhero-y!
These patterns, in short, are a GREAT resource and allow for a ton of customization. They are easy to use and easy to understand. For anyone interested in getting into to customizing these patterns rather than having to hunt down other patterns for a specific sleeve or collar, I’d also recommend picking up This Book. It’s an awesome guide on how to modify an existing sloper, or basic, pattern (Along with how to draft your own, if you’re that courageous) to be almost ANYTHING.
Enjoy and happy sewing :D