installing return air
I don't know of any online tutorials directed at DIY duct installation.
You could do what I did and take a 4 year sheet metal apprenticeship through your local SMWIA union, that way you'll not only learn the "how-to" but also the very important "why".
Installation isn't really that hard. If you get a copy of the mechanical code book that is used in your area it will pretty much tell you what you need to do to pass code as far as hangers, joints, materials etc.
The problem is that it takes longer to understand the code book and where to find the info pertaining to your situation than it does to put an entire mechanical system into a home.
"The physician can bury his mistakes, but the architect can only advise his clients to plant vines." -- Frank Lloyd Wright