@SarahGen-- If you want to temporarily curl synthetic hair extensions, you can dip them in warm water, squeeze out extra water and roll them in rollers. Make sure the hair extensions are off your head. Leave the extension on the rollers to dry somewhere and then remove them slowly when they're dry. They should be curled. When you wash them, they will return to their original, straight shape.
This is only for temporary curls, so it's a good option if you like changing your hair style often. If you want a permanent curl, then a curling iron on low heat (if the hair is heat resistant) will work better. I've never tried hot rollers, but that may be an option as well if it doesn't become hot to the point of damaging the extensions.