As a rule of thumb, set the flat iron to the lowest temperature that will still produce curls. A low to medium heat below 340 °F (171 °C) should suffice.
If you want sleek looking hair, make sure to blow dry your hair first, and straighten in sections. Always use a heat protectant on your human hair weave. Hair can burn and once damaged, it cannot be reversed. Make sure your hair is clean before your attempt straightening it.
If your wig is made of human hair or is a heat-resistant synthetic wig, you can straighten it with a flat iron. For most synthetic wigs, however, you’ll need a gentler method, like using a hair dryer or a steamer for medium- to high-quality wigs, or even just hot water for cheaper wigs.
It is not recommended that you wash, blow-dry and flat iron the hair in order to achieve your straight style. Wet and wavy hair is meant to curl up when wet, it is however recommended that the hair be completely dry when attempting to blow dry it straight or flat iron as mentioned above.
Flat Iron - Yes, you can use flat irons on your tape in hair extensions, making sure the heat setting is not hotter than 200 degrees Fahrenheit.