It sounds like you're asking how to cut a mortise? Are you a beginner? Or are you asking how to layout the mortise?
There are some video's on youtube. Timbeal has some good vids on his youtube page. Search "Moonhillfarm" on youtube and click to "see all" his videos.
If you are asking the basic question of how to cut a mortise, I always recommend beginners take a 2-6 day timberframe workshop. You learn a ton more from a workshop than anywhere else, and faster too. It's worth it. Cutting a mortise starts with hogging the majority of the material out with an auger bit using an antique boring machine, an antique hand auger, or a modern large drill and auger bit. Then you use your chisel to square up the mortise, and check it with your framing square and a machinist square. Though if working in the round, you probably won't be able to use the machinist square as you would with a square timber.
