What happens next?

How to make a plaster cast of an arm
Realize that you'll have to remove the cast.
If you are making your own plaster cast, you'll have to be able to remove the cast yourself. Removing a plaster cast requires having access to either plaster shears or an electric cutter.
It's highly dangerous to do this process yourself, so have a friend ready to help you. Plaster shears are safer than electric cutters.