Cutting carbon brushes should be avoided as it's too risky.
The best option is to sand or file brushes down carefully a bit at a time (using a fine a sand paper or a file) until they fit the desired brush holder of your machine.
There is no need to bed in brushes, as once installed they should be up and running immediately, with no need to wear them in at all.
New Motor Brushes Sparking
If your new motor brush is sparking, it's highly likely due to a poor contact or excessive friction between the brush and the commutator.
This is a sign that your carbon brush is either the wrong size, incorrectly installed, or there's a contaminant like dust/dirt/oil on the surface of the commutator or carbon brush.
Inspecting your tool should help you to determine which is the issue.
If you're unsure, you can always contact us at Top Deals Online, and we'll be happy to help diagnose your issue and recommend an appropriate carbon brush.