There are many different ways to install insulation. The pros often use spray foam insulation or blow shredded insulation. If installing shredded insulation, fill the attic space at least 15 inches high with insulation.
If installing fiberglass mats, start by laying the mats at the furthest point from the attic access. Roll the sheets out and cut them perpendicular. When you reach a wall, pull the insulation back over a joist, lay a level over the insulation and cut across the level. This will give you support to make a straight and even cut. Work your way across the attic until you are back at the attic access point. If you run into a cross brace cut a notch in the insulation and continue on.