Start by breaking your block or blocks of Styrofoam in pieces that fit into your glass container. Then take the pieces and push them into the acetone. Continue pushing Styrofoam into the acetone until the blocks you put in don't melt anymore.
Then wait one to five minutes for some of the acetone to evaporate. Wait one minute if you want it to mold the plastic and five if you want to shape it. The more you let the acetone evaporate the more solid it will be. You might even want to mold it while its still "liquid".
Here's a video that demonstrates the melting process.