Tips/tricks for vents in attic?

Never had to combine vents in attic before. Always went straight out roof with one stack. Now i have 3 vents going up 3 walls i need to combine in attic. Any tricks to this? Is it as straight forward as it seems? Flip a sanitee over, make sure there's 1/4" pitch on the horizontal? Since i have 2 lines that need to come to the third one that goes out roof, how high above the joists would you put the first horizontal? A couple inches or a foot? I've never seen this done in person, just in books, that's why i'm asking. Anything i need to watch out for?

Thanks in advance.

The vents can rest on the framing.

Yea, they need to pitch back towards the drain(s) but not necessarily a quarter inch per foot. Just enough to make them drain. Most guys use upside down plastic J hangers. I like to use strips of 3/4 plywood with holes drilled in them.

Also, the three vents do not need to connect at the same location.

Use lots of supports as the pipe can sag from the heat. I watched an episode of "Ask This Old House" recently where Richard had to redo a horizontal vent in an attic that had a sag which completely filled with water.

It is pretty much, "do it however it works best for you". There are few "regulations" regarding vents as long as they do NOT turn downward before connection together.