When one tube is mitered (aka notched or coped) to another, there's no single angle you can fillet-braze the whole joint at without moving it. It's not like sweating a copper plumbing pipe. You need to be able to use gravity to help rather than fight you.

That can be done in as few as two orientations if you have no choice, like flipping the part over once halfway through, but that's far from ideal. Ideally you want full freedom in three dimensions, to where each dab of filler is applied with the angle "just right", moving it frequently in small increments. How to actually do all this angle manipulation is a bit of an artform, hard to explain but like second nature after you've done it enough. Beginners should do a dry-run with the torch off, to confirm for themselves that they are able to easily get the joint to "just right" all the way around, with a minimum of faffing about (and letting the joint cool in the meantime).

For most FBs, that's done in a Park stand or equivalent, with a tube 1" or larger clamped in the jaws, jaws set loosely enough that you can rotate the part within the jaws by pushing on it, but tight enough to stay where you push it. Then the arm with the clamp is also free to rotate, on an axis at right angles to the first.

When the tubes are small, as with a rack, the thing that you clamp in the Park stand could be the fork steerer, if you're building a front rack, or maybe the toptube if it's a rear rack. When It's something that doesn't attach to a fork or frame, I'd still try to find some way of jigging it to a tube that can be clamped in a Park stand. That can be an ad-hoc thrown-together mess of steel wire, spring clamps, toe straps, bubble gum and duct tape, just make sure the tube clamped in the stand is long enough to use it as a lever for rotating the stand.

Do you know Paul Brodie's Youtube channel? He has lots of videos that show fillet brazing. I just went on his channel homepage and searched on "fillet", lots of hits, here are two that look good:

If a picture is worth a thousand words, then a video is like 30,000 words a second. He talks pretty slow sometimes, but I don't know of any better resource short of going to Niles Michigan for a Doug Fattic course.