I've had to practice a lot over the years cutting and installing. Longer pieces usually install best with a helper.
Couple of tips:
I would use measuring rods (two pieces of stock where you measure and make a reference mark) where you can instead of tape...more accurate.
Lay the piece upside down in the miter box and cut. The saw cut across the "thick" edge should be perpendicular to the moulding.
The cut section provides a line on which to cope a connecting joint. You just back cut deeply (usually more than a 45 degree angle) along the profile line of the trim. Little time consuming and takes practice, but makes for best fit, especially in uneven finished drywall joints.