[Travis Schuh]

Attachment methods vary based on what to parts you are trying to join. The trick is to use geometry to give you strength. Imagine you are making the parts out of paper (or even mock them up in paper), and go through the thought exercise of how they will hold up to different forces.

On the picture you have attached, I would at least overlap the two top flanges and add rivets there so that you have more than just the bend resistance of the metal keeping the parts at right angles to each other. Based on the strength needed, you might add more reinforcing features, but overlapping the flanges is a cheap and easy thing to do.

-Travis