I've been cutting my own units for many years now. I wear a layered cut with areas that range from 3 to 4 inches.

I highly recommend investing in a good pair of professional cutting shears plus a pair of blending shears and/or a razor comb too (--Sally Beauty). The blending shears and razor comb are especially good to have on hand to help with properly blending the sides and the back areas into your natural hair.

I cut my units down in stages over several days. I start the process by giving the system a quick rinse with a little shampoo added to remove the factory conditioner, rinsing well afterward. This is followed by a moisturizing conditioner treatment.

I pin the perimeter of the unit to a strofoam head with lots of evenly spaced T-pins. The first cut I make is a rough, even, layered cut that takes the hair length down from 6" to about 4 1/2". During the next pass with my shears I start shaping this rough cut into the final style by taking approximately 1/4" more off.

Next, I unpin the unit and throw it on my scalp to see how things are shaping up and to figure out which areas need more length taken off. Then I pin it to the styro head, comb it into shape and let it air dry overnight.

The next day I attach the unit to my scalp using a perimeter bond with tape. Then I proceed with the third cut -- usually another 1/4" or so -- mainly concentrating on getting the top and front areas close to the final length and shape. I keep the hair misted with filtered water during the cutting process. After this cut I usually style the hair into place and sleep with it attached overnight to help it "mold" to my head. (I find this step very important -- it really helps the hair to settle down and makes the final cutting and tweaking easier to do. It's still a little too long all over at this point.)

On day three I focus on getting the sides and the back exactly right. I remove the unit and put it back on the styrofoam head for this step so I can see exactly what I'm doing. I use blending shears and/or a razor comb to work on carefully blending the sides and the back areas of the unit. I work slowly during this step. I leave the sides and center back a bit long at this point on purpose to finish the blending on my head.

Then I reattach the unit with a perimeter bond again for the final cut/tweaks using the blending shears and razor comb to carefully blend the back and sides again. Getting the sides and back blended right is critical... I trim just a few hairs here and there a little at a time using the blending shears to finish up (this step takes a lot of patience). Then I style the hair, apply leave-in conditioner, and sleep in it again overnight.

During the next day or two I comb the hair back and forth and side to side checking the cut for overall density, length and blending. Using my razor comb or blending shears I usually make a few final tweaks. That's it!

I've never used a Flowbee or a Robocut, but think the process would be similar. The main thing to remember is to leave the system hair longer than you think at first, then remove increasingly smaller amounts of hair to arrive at the finished length.

There are several helpful videos on YouTube that show stylists cutting in systems. Just search "hair system cut in" to find them. There are also some great videos of haircuts using vacuum cutters too. You should check these out before you start cutting.