When cutting extensions, it is preferable to use a pair of regular blunt shears to cut-in extensions.

Do not attempt to cut these using a razor. You don’t cut hair extensions, as you cut hair tradionally. It is better to start off by blending the hair first, and then cut to the desired length.

Do not cut too much hair!
It is better to let the client go home with more length than originally planned to decide if they would like to wear it shorter; if you cut off too much, the client will always think they wanted it longer…