Great to hear from you again! Which costumes are you thinking of working on, exactly? My advice will vary based on that.
There’s no real rule on how you start your first costume, since it all depends on what your costume is and your comfort levels. Even when you’re a seasoned cosplayer, the decision to make vs modify is generally based on the difficulty of the piece and how easy it is to find something comparable in a real life garment. There are moments when you’re going to find a piece that’s uncannily similar to what the costume requires—reminds me of how Bisho mentioned stumbling on a crazy accurate necklace for her Eartha Kitt Catwoman—and there are others where you just won’t find what you’re looking for and will have to make from scratch—notably, Izzy’s boot covers. (Note: unless, you already have something in your closet, you’ll always have to just buy your shoes, or buy shoes and add covers to them. Unless you can actually make shoes, in which case, hi, can you make me shoes? I’ll pay!)
Let me break down some costumes to show you what I mean:
For Trish, I made the corset, armband, and belt, since you really just don’t see anything like those pieces unless they’re custom-made. The pants, on the other hand, were bought and painted—well, I had them made through a pants-making site because I’m a bit crazy, but long story short I did not make that.
Or you get a costume like Qunaribela, where it’s like: “Hey, gray leggings are easy, I’ll just do some paint and stitch detailing and BAM. There’s a look. Something like the boob top, on the other hand? Yeah, that’ll have to be made from scratch.
Or, on a more complicated and theoretical end, what I’m currently doing for Scarlett and Sheriff. Again we’re doing bought pants since they’re pretty easy (faux leather leggings for Scarlett to minimize the silhouette of my leg when the peg leg goes on). The leather duster and vest are also going to be made from leather coats we bought on eBay—yes, the vest is an incredibly cannibalized $15 coat because Beth performed some eBay blood magic on that one. The Sheriff’s hat is also going to be altered in shape, though we haven’t finalized how I’m doing to be doing that bit so TBD. That said, I’ve opted to actually make the Sheriff’s vest, just because I’m going to be doing sort of a vest extension that keeps the open coat in place, like how Tori made her AC3 MP Red Coat.
There are also costumes that are going to be entirely bought + modified pieces—think more realistic/plainclothesy type costumes.
Basically, there are some things that you can get away with altering, and the alterations will vary from the simple (Qunaribela’s pants) to the extreme (Scarlett’s vest). It all depends on what’s out there in the world, and what makes more sense for you to just bite the bullet and make. I tend to end up making more than I alter, depending on the scope and materials of the project—notably, if I can buy something pre-existing made of leather and alter it rather than making it myself, it tends to be easier and more cost-effective than buying the raw materials and building something from scratch. Again, it all depends on what you’re making!