My stock is usually about an inch thick. I set the mortise gauge according to the width of my mortise chisel, usually 5/16″. I line my mortise chisel flush at the edge of the stile. I lean on it to make a mark then move the chisel over one chisel-width. This time I lean more heavily, making a clear impression on the wood. This is the mark to use for setting the mortise gauge; no ruler needed. Line up the pins of the gauge with this chisel-strike and mark the limits of both the mortises and tenons with this setting.
There are lots of approaches to chopping mortises. Try one repeatedly and stick with it for several dozen mortises. Then experiment with others. Repetition is really the key. I chop a V-notch right in the middle of the mortise’s length, and keep extending that both deeper and longer. Check that your chisel is parallel to the outer face of the stile. It’s easiest if that means plumb, rather than having the stock tilted on the bench. For a cupboard door, the mortises don’t extend to the top and bottom ends, but are stepped in a bit so there’s no exposed joint when you cut the stiles to their final length.
I cut tenon shoulders with a backsaw, and split the cheeks off with a chisel. This is easy because I know my stock will split reliably. Sawn stock might require that you saw all the parts of your tenon. I angle the front shoulder a bit, undercutting it so it snugs up nice and tight to the mortised piece when I assemble.
The plow plane’s adjustable fence guides it along the framing parts to cut accurately aligned grooves for the panels.
The panel fits in grooves cut into the frame’s inner edges. I use a plow plane to make these grooves. If you don’t have a plow, you could cut the grooves with a narrow chisel. In that case, deeply score the groove’s position with a marking gauge and then use the chisel bevel down to carefully pare the groove. Patience required.