What works for me is focusing on characters in movies/tv series that I love.
Watching them is great, but when you start studying the costumes, spending time with the creation of them and walking around in them it brings a whole new level of enjoyment.
The costumes that have flopped for me are when I made something because maybe I looked a bit like the character and it was popular enough to be recognized... but I didn't love the character or the story or the costume so it wasn't as fun to work on or walk around in.
Once you find something you want to do, here are three principles I focus on when replicating:
Shape: the template/pattern should be made in something very cheap until you are satisfied. Sometimes I borrow from other patterns already there, sometimes I construct them myself, but it is the foundation of everything else.
Texture: the material you're using should match how the original piece flows/feels/holds up... Even if it isn't to start with, can can it be manipulated to simulate it: matte, pitted, shiny, flowing, ridged etc. Also, consider what you choose for the final principle:
Color: matching the original color and weathering of the costume (if there is any) is the final consideration. Dying, painting, bleaching, staining etc.
There are plenty of tutorials out there to draw from too, if starting from scratch is a bit daunting. Even if the tutorial isn't for the costume you want to make, you can still learn the technique and adjust it for yourself. I have a few you can look at if you want - in my signature link.