The easiest way is to remove the window and regulator so they are not in your way. Since you took one from another tail gate you probably know that already. If they got it for you, then there are a couple of clamps holding it in the bottom of the tailgate. When you get ready to connect it to the passenger side of the tail gate support, the easiest way is to take one bolt out of the loop it goes through, loosen the other. Bring the tail gate to almost closed so you can just get in to work on it, put the loop over it, re-insert the second bolt, snug it up with a ratchet wrench, lower the gate and do the final tightening of the bolts. That way you don't have it loaded while trying to get the loop on and bolted. I've done it that way several times, both removing the tail gate and installing it.
Hope that makes sense, if not I'll try to be more specific.
Good luck,

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