Use your free hand to brace yourself and work the saw blade back and forth with full strokes – “Let the saw do the cutting,” is a good reference here.
Once you’ve made your markings, rest the saw blade on the line at a 45-degree angle. Place your index finger on the side of the blade whilst you get the saw cut started, so you’re more in control, and grip the saw handle. Use your free hand to brace yourself and work the saw blade back and forth with full strokes – “Let the saw do the cutting,” is a good reference here.