To straighten a synthetic wig, the most important point is to control the straightener's temperature, you should keep the temperature below 300° Fahrenheit (149° Celsius), or the hair will melt, and stick together.
And here're the steps of how to straighten your hair.
Step 1: Put on your wig, spray some hairspray or apply some hair gel to keep the hair wet and soft, and which will also protect your hair and make your hair look shiny.
Step 2: Prepare your straightener, keep the temperature below 300° Fahrenheit (149° Celsius). Straight one strand by another, and from root to ends. And do not stay too long in the hair.
Comb the hair, and optionally cut the hair ends to blunt.
The finished look:
Please note that the curls will not revert back after washing.