My first mortises were made with only a plunge router, spiral bit and a simple edge guide. One thing that helpes was clamping a couple of of the legs I was mortising together to the bench for added support for the router base (you do not want the router to tip at all). Make sure that you only route in the direction that "pulls" the edge guide tight to the piece and take multiple passes of only ~1/4" at a time until you get to your desired depth. As someone stated earlier, using a router to make mortises can take a while, but they come out great.
Then you just have to decide if you want to square the mortise or round the the tennon. Personally, I think it is easier to round the tennon.