Start by drawing or printing the outline of the wings and pinning them onto a long piece of fabric. Cut out the fabric around the wings and then color the wings with markers or paint. Attach the wings to some foam to keep them rigid and you're done!
There's no specified length for winged liner. However, one rule of thumb to keep in mind is that shorter-length winged liner will look more natural, and medium or longer wings...
It depends on what site you're using. On Facebook: search for groups in the fandom, find one you like, send a request to join, then if accepted, post! On Twitter...
You can start by decluttering your room! Take the toys out or anything that looks too childish. If your parents won’t let you sell or donate them, pack them away...
It may just make your angel unique. You could add touches of it throughout the whole costume, if you like. You could also add glitter to make it less spooky.