Of course you need something to hold the tubes to your design. You can try to avoid calling it a jig but it is going to be something physicall to do that job. And you need some method of keeping the frame aligned. A straight edge with an adjustable screw will work. If I didn't have my awesome laser cut made-in-Ukraine fixture, I think I'd use a piece of plywood or MDF board to draw my design. The piece should be big enough to draw a wheelbase line to help easily determine BB drop and the center of the front and rear wheel. This allows you to measure to the bottom of your head tube (assuming you are going to use lugs) and to know how to get the correct angle for your chain stays.

Paragon aluminum tube blocks are the easiest choice but you could make some out of wood yourself if this was an exercise in doing it as cheap as possible. I'd bore the hole between two pieces of wood and then lay one of the half pieces face up on the plywood. It could be screwed or nailed in place. You would only need three, 2 for the head tube (use a long piece of head tube that stuck way beyond the lugs in both directions). The ends of the head tube could nestle in the cradle of each half block. And one for the seat tube sticking beyond the seat lug. Of course you could do 2 or 4 more the same way for the top and down tube to insure they didn't deviate even slightly from the design. But of course the lugs hold them in place so they would not be necessary unless they help with mitering accuracy and holding to spec.

I would braze the seat tube to the BB first. Then I would spot braze the entire front triangle together on the plywood. I'd put some sheet metal underneath the places being spotted. After brazing the front triangle I would braze the dropouts to chain stays and then braze the chain stay units to the BB shell using the T tool I posted pics of in a recent subject thread. Of course everything is in place and position to braze on the seat stays last starting with joining the top first. That way using a true wheel the length of the seat stays can be altered to center the wheel (I usually just file the dropout a bit). Those would be done one at a time to correct any length problems.

This of course is just a skeleton of concepts so this post doesn't get too wordy. Doing a fork accurately would take a bit more effort to make a fixture out of wood but since it is a hobby build and you are not on the clock, it won't matter how long it takes to build a homemade fork fixture.