The mortise, like most all woodworking joints begin with layout, at the right, you'll see that I use a marking gauge to layout both the mortise and the tenon, also note that before hand I chose the face side of the stock and will strike all the layout lines from that face, the result will hopefully be a joint that is perfectly flush on one face, and with any variation being left on the other...which will get scraped or planed flush later..

After the joint is laid out, I drill out most of the waste using a smallish forstner bit, just staying inside the layout lines, there are many ways of removing the waste, some like to use a chisel to remove all of it, some like to use a mortising machine, I like drill it and pare the rest out with a chisel. Shown at left is a pre-pared joint as it comes from the drill press...

A word about sharp tools, the shavings shown are from the end grain of the mortise, sharpening is a must have skill for a wood worker, I am a fan of the Scary Sharp method. 

After paring the mortise walls and cleaning the bottom, your ready to fit the mortise, try tapping together ( tapping not knocking-the-heck-out-of-it) if it goes together easily, snug but not to snug, or too loose for that matter, all is well, if your like me you will see that some fitting will be needed, normally I will shave the tenon a bit here and there ( it will have shiny spots on the tenon where it is binding) until it fits well, I read somewhere that you should be able to "knock it together with your hat" and these fit pretty good...

The finished mortise and tenon.