It's been years since I chopped my last "square" mortise, as opposed to the round ones that I use all the time in my chairs. Chopping a mortise with a chisel defies the imagination. It seems like it would be nearly impossible to chop a mortise that would be uniform in width and depth, not to mention going straight into the wood. But I'm happy to report that it isn't that hard at all. Of course a little practice doesn't hurt, but the key lies in using a chisel the exact width of the mortise and orienting the workpiece so that the mortise is vertical.
I use the wooden clamp to hold the post in place. The flat on the front of the post registers against the flat on the jaws and insures that the mortise is vertical. Another clamp hold the wooden clamp tight to the bench. You can see the initial chops that I made to establish the mortise. Once these chips are cleared, I repeat the pattern only deeper. It's important to have the bevel opposite where the chip is forming. That way, the natural tendency of the chisel to shift away from the bevel will help free the chip.
Here you can see the mortise after being dug deeper. The chisel on the benchtop is an old chisel that I reground at 90 degrees to the original bevel, so that it would be thinner than my 1/4" mortise. I use this to clear the chips and level the bottom of the mortise without affecting the sides.
Here I am marking the crest to locate the shoulder of the tenon. Obviously, hand chopping mortises like this is no way to do "production" work. But in this case, where I have 2 mortises per chair, it's 20 minutes of careful handwork that I enjoy, not to mention that improving my dexterity with the chisel can't hurt.