First cut the raw lumber to length. If you are using CB Designer, then use the top 'length' measurement. If you are designing freehand, then make sure you leave enough extra length to account for the multiple cross-cuts that will be made later (step 9).
Cutting with the miter saw
Check the length of your kept piece. Once you are satisfied with the length, place / clamp a stop-block at the edge to measure all other pieces to the same length. If using a shared saw like at Techshop, double check that the miter angle is set to 90 degrees, and the bevel angle is set to 0 degrees.
Place your hand on the kept-side of the wood, pressing it against the rear fence and the stop block. This supports the kept piece and makes a clean cut. Do not hold the off-cut side of the wood, which can lead to messy cuts and kickback by trapping the kept piece between the blade and stop block.
Fully extend the saw blade and turn it on, wait for it to come to full speed before lowering it into the wood, push forward through the cut.
When finished, wait until the blade stops before lifting the saw. Lifting while the blade is still spinning can create a messy edge
I used the miter saw because it was quick and available, but a table saw with a cross-cut sled or handheld saw can also be used to make this cut.