Before you straighten a wig, answer one question: what is it made of? Human hair can usually handle heat like natural hair. Synthetic hair may melt unless it is heat-resistant.
Human hair wigs
Use heat protectant, work in small sections, and keep the flat iron moderate. Do not press the lace or knots. Let each section cool straight before moving it.
Synthetic wigs
Check the heat limit. If you cannot confirm it, use steam or a hot-water method instead of a flat iron. Test one hidden strand first.
For frizz plus waves
Detangle first, then smooth. Straightening tangled fibers bakes the knots into the wig and makes breakage worse.
Bottom line
Straighten human hair with controlled heat. Straighten synthetic wigs with steam, hot water, or very low heat only if the fiber is heat-safe.