It's about being like a walking decoration.
There's a lot of layering...Like, two tutus over leggings with tall socks and legwarmers. Hair accessories are really important, you have to have a lot of them. The best way to do this is to put them in your bangs. Often, you wear pigtails.
The idea is to have your entire body accessorized, so it's also great to wear fingerless gloves, and bracelets, and things. There's dark decora, pink decora, and rainbow decora.
Something you can do is accessorize with things taht aren't meant to be worn. Like, attach candy wrappers, stuffed animals, and toys to your belts and necklaces and such.
A lot of decoras, like myself, want to have playful hair as well, so we use wigs, so we always have hair that goes with our outfits.
Hope this helps!