Hi i was just wandering how to make costume skin and what you need to get

(i know you need rainbow jar) But what else do you need and what are the steps

All you need is a Rainbow Jar. Once oyu have that, you go to your inventory and select Use. This will bring you to a page where you can select which pet you want to use it on, preselected will be your active pet. When you have found the pet in the list, click Paint. Now you can choose which skin would fit your pet best. 

That's how you put a custom skin on a pet; if what you meant is how to create your own then look here http://beastkeeper.com/new-color-upload

thanks but i dont understand it there ask for a png pic but they would not accept my pic still need some help

You need to save the custom skin as png if you are finished. They have to be without background so only the animal is showing. If you are unsure about how to do it this might help :>

thanks

Did you manage to get it to work? PM me if you need help getting it to work. I'm happy to assist!