I first drilled through the slab with a forstner bit to take away a lot of the material, this was not easy as the slab is very heavy and trying to keep it steady on the drill press was a challenge, in the end I had to make a set of legs the right size to stand the free end on. Also the bit was clogging up very quickly.
I then made a jig the size of the mortice and routered the square holes as deep as I could using a bit with the bearing on the top. The slab is about 80mm thick so the bit was not long enough to go right through. After this i chiseled out the corners and turned the slab over and used a bit with the bearing on the bottom which used the cut already made as the template to cut the remainder of the material out.
I then squared out these mortices and chiseled out the sides on an angle in order for the tenons to take the wedge when it comes to assembling.
-- "These are my principles. And if you don't like them, I have others." ... Cheers, David from Sydney Australia