If you have straight hair and want to wear a synthetic extension, add shine serum to the ends of your hair before applying the faux strands. This sheen the serum will give will help your real hair match that of the hair extensions, since they tend to have a silky-plastic look to them.
If you have human hair extensions, you can do whatever you do to your own hair—and the faux strands, such as flat iron or curl them and add shine spray. Just be cautious to protect the hair from the hair extensions as you would your own to make them last longer. They need the same TLC, and make sure to get them cut to bend your hair and trimmed if you are one to wear them on a regular basis as the split ends if they let run wild will tangle in with your own hair and WILL cause breakage and ratty looking hair. For more tips on long hair please read https://kerridawn.wordpress.com/?s=long+hair.