I like to start with the Head tube down tube join. Its the most important. Some builders start with the Seat tube Bottom bracket first, especially when doing a full fillet brazed or TIG build. (Paul Brodie build order here)
After the head tube I like to do the seat post bb joint. However before you can do this you need to prepare the seat post. I wanted to do a retro style reinforcement and a forward facing seat clamp.
Normally if I was doing a fillet bottom bracket I would do that joint first then do the work on the other end of the seat tube. Unless the bike is going to have a lugged Bottom bracket shell.
I got some 0.5mm tubing with a ID of 28.6 and cut it to shape by hand. I then lightly sanded the seat post and made sure this fitted correctly. It is tempting to make this a super tight fit but you need to make some space so that the brass can flow between them. So it should fit perfectly but slide off with no effort at all. For brazing of that you need to use a bigger tip again on a soft flame and heat the whole area. I like to add the brazing rod to the end facing the middle of the tube and use heat to pull the brass down to the open seat tube end. This way you can tell it’s come all the way down and it’s easy to clean up.
I also wanted a forward facing seat clamp. Which is find but you have to have to know which way is forward (so you need to have mitred the bottom bracket end). Also you have to braze the seat clamp on now and then cut your slot and drill the end of the slot to ensure it does not crack. Job done. if you are doing a regular rear facing you can do this last.
On a lugged build I would then normally start on the chain stay so I can then do the bottom bracket shell in one hit on a lugged build and on a lugless build I would do the front triangle first. I would normally start by tacking the bottom bracket to the seat tube, then tacking the down tube to the bottom bracket then the top tube to make the front triangle.
So to make the front triangle you need to block the head tube flat on the table and then lie the seat tube to it is 100% parallel. (tube blocks help). Check the down tube / seat tube bottom bracket joint. Then slide to the bottom bracket is off the table then tack weld. you can then slide back on the table and check from both sides that is 100% straight, then you can finish the joint. then you can pop the top tube on and check the whole thing is straight before you add the back end.
Once the front end is done I cut the dropout for the rear stays. Tack the chains stays to the Bottom Bracket. You can put a wheel in to make sure you are square. The you can do the seat stays. You can then put in the brake bridges etc.Then you just need to fit the cable runs and other braze ons.
You then have to scrub off all the flux with hot water to get rid of it then a good sanding or sand blast.