What works for you is right. If what you do works, keep doing it. This is just what I found works for me - no right or wrong way.
some things to try. Wayne stands over his material, I''m on the floor and kneeling over it. So you can put more even weight on the blade and keep it closer to your body, than if you were sitting and the paper was on a desk. -
Metal ruler with some thickness - the one I use is heavy and thick. You can get them at HomeDepot/Lowes etc. and if it's too long you can easily cut it in two with a metal blade in a jig saw.
T squares didn't work for me. Was hard to line up ' cause slight jiggle gets it out of square. and the short side of the L gets in the way of the cut. Using a T square was a big part of the problem I was having.
With the snap-blade utility knife a sharp blade is always just a snap away. Before I would try to get by with a dull blade, cause I didn't have a sharp one handy or a stone and I'm lazy. Same with the cutting mat - I would use a piece of cardboard etc. instead of a cutting mat.
The 75 degree thing is just what I found works for me. I hold the knife straight up and down against the straight edge, but lower the handle to about 75 degrees - or 15 degrees off perpendicular - most people say 45 degrees but I found a steeper angle helps me keep the tip of the blade on course - but whatever works for you is right.