As this section does contain a major spoiler, I decided to make a subpage for the main article.
As a special detail for Mayu's costume, you might want to consider the crimson butterfly bruise, left from the Strangling Ritual by Mio.
This bruise that the butterfly represents is made from the imprint of two hands closing over the neck. The fingers wrapping around the neck, and the thumbs can occasionally be the head or antenna.
Mio, in the Crimson Butterfly Ending, also has the same mark as The Remaining.
Now, I'm no expert in make-up (but hopefully soon I can get a proper written tutorial) for bruises, but youtube usually offers a lot of video tutorial on make-up techniques. The butterfly bruise is a good detail for cosplaying, so its a good thing to consider.
--Dio
(pictures from game.pchome.net)