Any number of ways, so I'm sure more will follow, depending on what works best for you. More often than not, cutting same lengths that matter, say legs, sides of a box, whatever, I would use stops clamped to the miter or the work surface. Once in a while I want to see exactly where my cut will be on the table saw, so I lay a piece of masking tape across the table, tuck it over the ends, because it does not stick well to a waxed surface, meaning you have to be careful and can't dilly dally, then cut a piece of scrap with the miter, bring it back, and mark the cut edge to the tape. If "free handing" it on the table or miter saw, I find it easier to focus on a single mark, right where the blade will first contact the wood, rather than a line. If you do want a line, minimize the steps to reduce error. Rather than measuring the piece, making a mark, and then laying your speed square to the mark, hold your speed square to the piece, measure to the edge of your speed square, and draw your line. Might sound the same, but it's one less step, so one less place to be off a bit.