I dug out my spare tailgate to make an example out of it. Poor tailgate
So you can see that once the carpet is pulled off there is a very easy cut line to follow.

Drill a starter hole.

You can see the divots where the 3 ribs are spot welded in with 3 spot welds each. With a normal length jigsaw blade you can cut right through the ribs if you go slow.

Cut along the line making sure to remove the entire raised portion.

There is one section that you can not cut with the jig saw, if you do you will cut into the latch. I just use a cutoff wheel for that little bit, or you could block up the jig saw a bit, or use a shorter blade.
If you remove the inspection cover you can see the problem area beforehand.

A shot of the interference once the center has been removed.

Once the center is pulled out, remove the ends of the ribs by tweaking them back and forth until the spot weld breaks.

Pile of scrap metal left over.

The big hole in your tailgate.
I do not recommend completing this step unless you have the requisite box to fill that hole. :-)

And the hardware, note the lock washers, do not skip the lock washers.
If you do, I guaranty that the bolt will vibrate out.

The hardware would be tough to get together unless you send the 1/8" allen wrench through the hole, line the washer, lock washer, and nut up on the allen wrench and use it to aline them to the hole, pull out the allen wrench while holding up on the hardware and thread the screw in. Makes quick work of it.

No really, this would seem nearly impossible if you do not use this trick.

Tailgate Storage Lid Installation

All set