I find most of my tutorials on Pinterest, so you can check out the full and totally unorganized list here, the rest I've tried to put together in some semblance of organization. Enjoy, and if you find anything that you think would be good to add here, let me know!
Props and Armor
This is essentially how I make half of my props these days, so a lot of the information here will look familiar!
Cardboard is great and inexpensive, and fairly sturdy, plus you can do a lot of stuff with it. There are several tutorials under this one so I'll organize it by what they're making in each tutorial
Reaper Wings (this one's actually mine)
Various Props (there's a lot of good stuff on here with using cardboard for a variety of projects)
Another awesome and inexpensive material to make a variety of things
General EVA/Craft Foam Tutorial (This has a lot of good general info and where to purchase materials)
Shaping (how to achieve curves and other shapes in foam, there's some worbla stuff in there too)
Tooling (engraved designs in craft foam)
Resin/Plastic Casting and similar techniques
Fabric, Sewing, Etc.
If you're new to sewing, there's a great collection of knowledge and tutorials to get you started here. It has everything from patterning to sewing buttons, using different kinds of fabrics, and all kinds of useful things! Thanks so much Katie, Jan, and the other young ladies for suggesting it!
Drafting a Pattern from Existing Clothing / Drafting Tutorial B Both of these tutorials show you how to make a pattern without taking the existing garment apart, which is great! You can also go to the thrift store and find something that fits, is cheap, and *can* be taken apart to use as your base pattern (that's what I usually do).
Using Single-Fold Bias Tape - Single fold bias tape will give you a clean edge around an arm hole or other raw edge without giving you a strip of color along your outline. This is useful if you're not lining your costume.
Using Double-Fold Bias Tape - Double fold bias tape folds around both sides of the edge of your fabric, so it will give you a stripe of color along the edge where you add it. A lot of anime designs have colorful borders along the edges of garments so double-fold bias tape is a great way to achieve that.
Installing Welt Pockets - Welt pockets are the small pockets on vests and similar garments where a pocket is added to a solid piece of fabric, rather than a traditional pocket where it's added in along a seam. Lots of designs have them so it's a useful thing to know how to make, because real pockets on cosplay are always helpful.
Armsocks - A useful tool for strange skin colors on your hands so you don't have to paint them.
Dying Fur - Finding the proper colored fur can be difficult, and fur can't be dyed in the traditional sense, but it can be done with this method!