I have one of those electric [plug-in] hair trimmers ..a good one with a wide cutting edge/ front. What I used to do when I wore a topper and my own hair needed trimming/shortening was I `back-combed `the hair[with the system off the head] so that it stood out almost at right-angles to the head. Sitting in front of a mirror and using a hand-held mirror I could see the back... I just trimmed a little at a time as evenly as I could....the nape hair at the very bottom is the shortest so don`t trim that up too much at all..just go easy there [maybe 1/4 inch at a time]....the hair up near the crown was generally a lot longer so that was the hair I trimmed in shorter.... visualize a side-view of the head with all the back hair standing out almost parallel with the floor.....the bottom / nape hair is relatively short while the hair gets longer as you `go up` towards the crown.... I trimmed it as evenly as possible so that the `top`hair was almost as short as the nape area.... when you comb it back down it`s pretty even and not `gapped` ---you can do the same at the sides..... just take it very slowly at the start..... at least your own hair will re-grow if you go a bit `heavy` as opposed to a system which once it`s cut, it`s cut and that`s it...... my hair was quite thin and `fine` so that`s what worked for me as I didn`t want to be going to a stylist with a topper on and asking for them to `be careful`/ go `around`it in order to cut my own, so I just got into doing my own...got quite `handy`at it after a while..... it`s best to get a good/proper cutter with a nice wide front/blade area as it helps to keep more hair cut to the same length with each cut.....once with only a narrow front will tend to leave a `gappy` effect... paul.