Check the quality level of the synthetic hair you have.
The cheapest synthetic hair cannot have high heat but keep the heat as low as it can get! You don't want to have the risk of burning your hair and the hot rollers used on it as this will most likely melt the synthetic fibers. There are, however, some kinds of synthetic fiber (e.g., Monofiber, Thermo fiber) that can withstand heat to a certain degree. If the ability to use heated styling tools on the extensions is a necessity for you, check out this possibility before you buy.
Use detangling spray on the extensions daily.
Always comb this through completely. This product should stop the hair from tangling and stop it from getting frizzy. In addition, use an appropriate comb or brush designed for use with extensions; normal combs and brushes can damage the wefts and cause the extensions to loosen or pull out.
Wash out any hairspray or other styling products at the end of the day with a mild shampoo and conditioner.
Use cold water. Invest in some shampoo designed for synthetic extensions. They are made of the same kind of fibers and this will help stop your extensions from tangling.
Trim frizzy ends then rub some oil onto your extensions and comb it through.
Leave it for an hour then use a towel to wipe off excess oil. Wash with a mild shampoo and rinse.
Make a decision to renew or remove the hair extensions well before they look ratty.
Hair extensions tend to move up the hair shaft at around the six week point, making them prone to loosen or become uneven. If this is your first time wearing them, you might decide that the work involved in caring for them is not worth keeping them again; or it might be something you're willing to keep with for a time longer. Either way, a return to the hairdresser to get them sorted is essential at this point.