It depends. If there are character sheets for it then I save them and look at them of course. If it's from an anime, caps are always nice. If it's from a game, I either replay the game or try and get screens from there to from angles for coloring and style(hair). During caps and references, I plan out how each can be made and see what other's people approach on it is if I'm stuck or not sure if it will work.
From there, I make a rectangle and wing it. Hahha I have yet to follow a pattern.

Look around at people who've done the same costume- sometimes even send them messages on coscom or dA or wherever and ask them specific questions.

If I'm doing a costume where I can't find a stellar example already in existence, I usually try and decipher what each piece of the costume is supposed to be and look up general tutorials for putting it together (e.g. hoodie/shorts/sleeveless whatever tutorial).

Take a million screenshots. Usually put together a sheet with angles from all sides and close-ups on small details. Rewatch scenes where stuff blows in the wind or is underwater if there is any and see how it reacts. (Of course, some shows will fuck with you on this one, like in Digimon where Sora's hat is a helmet that makes a clunking noise when hit in one episode, but then is crumpled up and throw off like it's made of fabric in another.)

Once I feel confident of what everything is meant to be and how I can go about making it, I go for it.

I draw out the costume in all of its layers. For more complicated stuff, I might have a bunch of pages, breaking down the pieces and studying how everything goes together.

Screenshots screenshots screenshots. Get episodes/cutscenes if you can, better yet if they're in high quality. If it's a character from a game, master your Google-Fu and try to find some character model dumps, or at least high-res renders. 3D modeling sites are great for that sort of thing.

Pictures of other people in costume are alright, but remember they should be second priority to the character image itself. No costume's perfect, after all-- everyone's bound to make mistakes, if you're using the cosplayer as the main reference you don't want to end up mimicing them.

>>4843154
Looking at the original source is vital for all the cosplays you do. Take or search for captions in all the angles possible. If the cosplay is too hard to translate into a human, search for the best cosplayer of that specific character and take that cosplay as your main guide. Try making some changes if you need to.

When I work on a costume I usually seek out images of the costume online. First from the original source material, then from the cosplayers themselves to see what improvements I can make.

I like 3D models; You can easily expand or remove parts, and if it's armour, you can use it to make a paper model - paint it with resin, glass it, paint it, bam - functional, accurate armour.

I always go for a pic search and save as many pics of as many angles of the outfit as possible. Then I go and research other cosplayers. This is to see who used what material; which one looks the most effective; and hopefully sometimes they write how they went about the outfits.
If all else fails, I sit at my tv/computer and draw out details (had to do this for my Gracia cape as no one had properly done the decal on her back).

From here, I take apart the outfit piece by piece. I measure myself and work out ratios to ensure accessories or featured parts aren't too big or small.

From here I'll take a trip down to fabric stores and see what selection I have. If I'm not satisfied; online shopping I go next!

I used to ask cosplayers where they got the material if I can't find it in fabric stores, but most of them will not reply or be rude (as in tell you to look yourself) So I spend hours searching instead. It would be so easy if it werent for the fact I was in Aus (and most online stores WON'T ship here), which is the only reason I ask. I hoped someone would help me with pleather; but no one has bothered helping.

It may seem like a lot of hard work for something so unimportant; but a lot of us who love the hobby, want to do characters justice.