Objective: How do you put insulation in a wall?

Which of the following solutions is more sound in terms of naive physics reasoning?

OPTIONS:
a). Be sure the gaps between the board match the width of the insulation, use a pencil to cut the insulation to the correct vertical length, then press in by hand.
b). Be sure the gaps between the board match the width of the insulation, use a blade to cut the insulation to the correct vertical length, then press in by hand.
b).