Don the most common mortising jig is just a slot in a piece of mdf or plywood and you use a guide bushing to ride in the slot. For example you could cut a 3/4" slot in a piece of panel board with a 3/4" bit and and edge guide and then use a 3/4" guide bushing with either a 1/2", 3/8", or 1/4" router bit. Since you are using a floating tenon the rest is simplified a bit (regarding cutting the tenon with a router bit at least). The only issue then is the registration of the jig to get the mortises where you want them. Just as an example, if you were mortising table legs and the aprons that would fit to them, then the easiest way is to offset the aprons slightly inside the edge of the table legs. This is done for two reasons. First is that it looks better that way, and secondly it isn't critical that the mortises line up so that the edges are even which is extremely hard to get perfect. So the second job of the jig is setting the registration and if you plan properly that isn't a big problem and allows you a lot of leeway in how to do that.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.