MY best advice is to get the d/s positioned and snug (not tight but snug) then go to the pass side. Get it snug then put the rear bolts on the cat back. Snug those up (again, not tight). Then go back up to the d/s and double check your position on the flange (ensure proper coverage of the ball and socket joint) and tighten it down, move to the pass side and tighten, finally tighten the rears up too.
I find it best to put a jack under the x or h part and jack the mid pipe up to where it should be (I don't use the hangers, never have) that way you can ensure proper alignment and a leak free pipe from the get go.