i do my own cut , not too difficult

first you get something proper for the unit to sit in, such as a doll head
before you cut, you need to make sure you get the proper tools, such as a sharp scissors, sharp thinning scissors and the a good comb
you need to wet your hair and comb the hair straight first, thats how you can measure it properly, wet the whole hair
start measuring with your index and middle finger , grab a bunch of hair and start cutting.. leave some room for error because wet hair are usually longer
once you make you first few measure / cut, things are a lot easier , simply repeat the proces

you may rather cut it slightly longer than you wanted to begin with, that gives you room for any errors, then do a further cut in after wearing for a few days, if you find that is necessary