So, I got the table legs cut (and rabbet joints in them). Now to attach
the top frame to the legs, how do I make sure that the individual
member of the frame gets attached exactly 90 deg. to the leg?

I mean.. it is hard to hold down the leg and frame and then put a screw
so that they are jointed at 90 deg.... even if they are clamped down...

In general, how is the assembly done? face down? but then once you have
two legs joined to one member of the frame, how do you assemble the
pair.. there is no 3-way 90deg. clamp available..

it is hard to explain in words.. but if you get the gist, how do you
assemble cut parts where one part is laying horizontal on the table and
the other part is to be joined 90 degree vertically, to hold it, square
it and then put to drive a nail or a screw..

i guess it cant be done unless there is another person helping or there
is a horizontal-vertical clamp?

Thanks