Always did this by careful use of a sharp knife - take care or you cut more than just nails...
Did this so often on car wiring : 14/010 etc...
The best way I found was to place the wire on a wooden block then with the knife blade at the point of cut gently roll the wire : light pressure goes through the insulation and you get a nice even cut - with less risk to fingers...
Then for the middle waste bit - it is possible to angle the blade to cut a helix and the waste comes off easy - no flexing... takes practice though... :) Having had to repair burnt wiring looms in cars had plenty of practice...