One of my watercolors, with a decorated, or French mat. I do mine traditionally, with watercolors and inks; all hand done. I layout the cut mat with faint pencil marks, full for the wash area, and corner L marks for the lines, as an L mark gives stop and start points. I've tried various methods of masking the wash area, and that works for very large mats if one is using an airbrush, but for the most part, doing it by hand is the easiest and quickest. One tip for getting an even tone is to do a first wash of clear water. This is a task that rewards practice, though as the wash area is usually surrounded by ink lines, there is some "wiggle" room.
I use a draftsmans ruling pen to do the ink lines; I've used the Rapidograph type pens, but they are generally more work to keep functioning than the two blade ruling pen; my favorite being the jack knife style with an articulated joint for easy cleaning. I can alter line widths and colors very rapidly with the two bladed ruling pen. The pen should be filled with a brush or eye dropper, checked for drips, and tested. No dipping, it's rude.
The straight edge needs to have a "stepped" back, to keep the edge above the line and away from the ink. Cork, as seen on commercial rulers, or even several layers of masking tape works. This is still free hand work, aided, but practice is needed.
Aesthetics. This is the tough part, but I've seen too many mats that overpower the work they are supposed to enhance. This is the true "ART" of framing; doing your best work in such a way as to not be noticed, while enhancing the art object. Washes must be subtle, washes, you know. The lining needs be subtle, muted and diluted, inks and colors.