Its pretty simple, I have been around an actual install and seen plenty of stack kits installed on trucks-
If you did not buy a stack kit and only the stack, things may be a little more difficult (depending on your fabrication skills and availiable materials).
To start, you will basically want to figure out where the exhaust passes under the bed when it is going straight back to its normal exit. Once you have done this, take a 5" hole saw bit for any kind of hand held drill, and take it to center of where the pipe will be coming up (I witnessed the people who did my buddies cutting it with a plasma cutter but most readups I see are done with hole saws and then trim with sawzalls).
After this is done, I would say have some 3 1/2" flex pipe (this may be a time to upgrade your exhaust to a 4" turbo back, though try and find somone who may be able to sell just a downpipe, then the amount of straight under cab pipe you need to get to where you 90* up) and bend it to a 90 up into the bed. Once in the bed, have a piece of whatever size pipe you are going to run into the stack, and go from the flex pipe to the base of the stack (measure for height of stack, where you want it, etc at this time). Cut to length, get things measured and clamped in place, see how things look, etc.
Once this is all done, check fitment of plumbing under the truck up into the bed, and THEN check the fitment in bed of whichever pieces are there (including stack). Once things are to spec how you want them (oh, and for last steps, get the stack clamped in place with the pipe you are going to weld into it in place as well for all the fitment checks) go ahead and weld up.
I hope this all works, as this is pretty much how I witnessed my buddies single stack being installed, and I would do if I was going to put a stack on my truck. Best of luck to ya man, and I am sure others will chime in as well, just give this a couple days.
When i did mine all i had to buy is clamps and adapter. I took the hump over the axle and cut a slice out and made a elbo and welded it shut with my mig. Time consuming and only saves a couple bucks. As for the hole you will have to cut out some of the bottom cross member for the bed. To hold it in place i used a u bolt and bolted it to the top bed crosmember. Go to link in sig for pics.
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