How do i prepare these mushrooms for cooking?
For heavens sake, DO NOT EAT THEM! You do not know if they are poisonous or not! I am a professional chef and WILL NOT eat any mushrooms that come from outside, it is too hard to tell, even from me and I have seen and prepared soooo many different types of mushrooms. You need a professional to look at those to certify what they are. You can kill yourself or cause serious injury to yourself or to anyone else that eats them.
Just wipe the loose black bits off with damp paper. Remove the frilly white bits under the cap and cut off the base of the stem to check for worms. Worms tend to occur in wild, older mushrooms. Discard any with holes in them. Slice them or leave them whole and do what you would normally do with them.