If you have a square sided leg and a straight apron, then the trick is
to lay everything on the table. Have the apron running along the edge
of the table and leg running across the table. Place the appropriate
thickness of wood under the apron to raise it to the proper location.
For clamping you have several options. The trick is to keep the clamps
out of the way of the screwing operation. I typically have two pockets
in the apron so I make a caul, just a blank piece of 4/4 about 3 x3
and I stand it on edge, parallel to and between the two pockets with a
clamp over it and under the edge of the table. I also typically setup
both legs so I can also run a bar or pipe clamp from leg to leg in the
same plane as the table over the top of the legs. Bessy's with long
jaws work best to keep the legs pushed into the apron without wanting
to splay out.