Okay so a while ago I bought fly and one of the cosmetic packs but I'm not sure how to use the pets disguises and particles and nobody will respond when I ask. Also- My fly isn't working anymore and there isn't a clear reason.
So after your receive your cosmetics and fly, to enable fly you just do /fly, to disguise as a mob, you can do /d <mob>, for more information just do /d. If you bought fly, it can take a while to come in, so try to also relog and join back in, it should work. You can do /menu to check for more options about particles and pets.