I'm making a Glee cake for my niece. I already have the design down, but I want to add a gumpaste slushie cup. I've only done flowers with gumpaste so I'm not sure how this would work. I'm planning to buy a small pack of Dixie cups and unrolling one and using that as my base to cut out the gumpaste. Then I am going to roll the gumpaste around an uncut cup and let it harden. I'm hoping if I coat the cup in shortening my gumpaste cup will just roll off. Does this sound like it would work? It sounds so simple, but I'm nervous since it's my first gumpaste anything I've made without a tutorial to follow.

Sounds like it would work - good job

You can also try putting the rolled gumpastes inside a cup and put another cup inside this first cup and press the gumpaste against it, kinda like a mold. After drying a bit - I think removing the first cup while keeping the second cup in will allow you to see if all the details came out, any rip or tears and then let it finish drying upside one on top of the 2nd cup... HTH

Why not just roll out the gum paste and cover a cup you want to use (like you would cover a cake), let it dry, and pull off?

I would not use shortening. I find that it keeps the inside from drying and kind of suctions to the mold you are using making it very difficult to remove without damaging it. Use powdered sugar instead

Thanks for the advice! I'll be sure to use ps instead of shortening.

I've done similar things in the past and always use powdered sugar instead of shortening. I wrap and tape a length of waxed paper around my form, apply the gumpaste and let it dry. When the gumpaste is hard, it will easily come off, take the waxed paper out of the gumpaste cup and you're done!