With a woodworking chisel and a hammer ! It may seem a little brutal but it works like a charm.
I wrap a small piece of masking tape on the header. I mark in between pins with a pencil, a little off to the waste side. Then to the garage bench, a couple of wacks on each side over a wood block and it either just breaks or some bending will do it. Then a bit of cleanup with the utility knife. As always there is one pin wasted, but it is quick.
My previous method was using a small, very fine toothed, X-ACTO saw. It works great but takes a little longer.
Tony