a few hours before.  For the right consistency, use about 1/3 cup (80mL) of water per packet of gelatin.  Color the gelatin.  Use a few drops of food coloring for an unnatural tone, or add a bit of liquid foundation in a tone close to your skin for a flesh-like look. Cut the gelatin into cubes.  Store it in a bowl or resealable plastic bag. If you heat it to boiling, you'll break down the structure of the gelatin.  Put it in a bowl in the microwave and heat in 10-second intervals, until the cubes have softened and become slightly gooey.

TL;DR in Chinese:
在几个小时前使用明胶。 轻轻加热明胶。