The secret to jigless framebuilding is to think of supporting each join in position, rather than of supporting the whole frame. Use the lugs to locate the two tubes against one another, and use a simple flat surface (my flat surface was some 40mm square section aluminium tube) to hold things in plane.
My 'jig' was my 40mm square section aluminium extrusion, a handful of pieces of sheet metal of various thicknesses to space things, a long 10mm bolt, and a couple of F-clamps.
Here's the general setup for the first join:
The aluminium section is simply used to hold the head and seat tubes at the necessary angle.
The second join:
Here a spacer is needed, to account for the width of the side of the BB shell. The hole in the extrusion is used to bolt the extrusion to the BB face. A large washer is used on the other side.
The third join:
Here the seat tube is joined to the bottom bracket. The extrusion is used to hold the seat tube accurately in plane with the head tube - a thin spacer is needed to account for the difference in tube diameters. I'm assuming you're using OS tubes, with 31.7mm head tube and down tube, and 28.6mm seat tube. The difference in radii is 1.6mm.
Joins four and five:
Here the top tube is added. I simply keep the extrusion in place from the last operation to ensure the seat and head tubes stay in plane.
Joins six and seven:
This operation adds the chainstays. This one is hard - you've got to ensure both chainstays are in the right place - I found it easiest to add one stay, then use a known true wheel to locate the second one. Pass the long bolt through the extrusion, and secure it in place with a nut. Then use a second pair of nuts to hold the dropout. This way by turning the nuts, you can easily adjust the width.
After that, the seat stays are added, and you have a bike.
