Set the log up so that your mortise will be plumb, take a drill and rough out your mortise. You could attach a bullseye level to the drill to help out. Then clean out the rest of the mortise with a chisel. Use a torpedo level to check your sides.

As for the tenon... if you are comfortable using a chainsaw you could cut the tenon all with a chainsaw. If not, break out the handsaw. Now if the post is square then you can use a circular saw all the way around the tenon.