I have a custom subclass for a druid and it has a custom creature that they can wildshape into. I was wondering if anyone knows how to make a custom creature or turn a custom monster into a creature so I can have my druid wild shape into it

Create the homebrew creature (look up how to homebrew monsters - to long to describe here)

to add it to your sheet;

toggle homebrew to on in the character sheet editor (home tab)

go into your character, then extras tab

manage extras, under add an extra choose category “misc”

type the name of the homebrew creature and select add and it should add it to your list