I have never pinned. I built my tandem and a lugged frame without a fixture. I had fixturing to keep the bb shell perpendicular to the seat tube, but it's not essential. Then build the hockey stick (head tube/down tube).
I always made a full scale drawing of that joint to get that angle right. A full frame drawing would be nice, I suppose you could draw one up and have it printed. I didn't have that luxury back in the '70s. Then when you join those two assemblies, the mitered top tube serves to tell you that it's assembled properly

Tack, measure, align. I think the main thing you need is a decent straight edge that's long enough. I think at that time I had a 40" ruler I used.

Having said all that, there are some simple fixtures that will help a lot. For example, Doug Fattic's t-bar. You should also be able to get some ideas from Paul Brodie's youtube.