How do you mold melted plastic?

The industrial shaping of melted plastic is almost exclusively done in molds. The process is called injection molding. A screw drives feed stock through a heater, and the molten plastic is forced out the "business end" into molds. It is possible to mold plastic by hand, but precautions absolutely must be observed. The plastic is hot and the fumes given off by hot plastic are toxic. Additionally, heating the plastic is something that must be carefully done as you can overheat it and pyrolyze the material. You can also cause it to catch fire. If you are not at ease with any ideas here, you should not be trying to mold melted plastic. Set yourself a learning curve and get up to speed before attempting anything like this. Safety is your first mission.