For ST to BB shell I don't use a jig. I have two short bits of angle iron welded together in a sort of W. Put that on the table, stop it sliding around with something heavy, rest the BB shell in it, and support the ST just on a welding magnet going into it. I check square and that it's centred by using the depth gauge on a vernier caliper. Tack, check again, and then completely weld that out.

Then this is how I fixture the seatstays:

New jig design

And I completely weld those before going any further, but leave the DOs tacked just in case (although I have never had to move them).

If the BB shell is square and centred and the SS are right it should all go well from there. I then use the minimalist jig described in that thread but you probably could manage without one.