Easy way is to open one of the rear doors (on a crew cab) and pull the rubber gasket off around the top of the door opening towards the back of the cab.
That will expose the edge of the headliner.
That should be able to be -carefully- pulled down a few inches. You'll be able to look up and see the roof skin.
Here's how I did my 2018 F-350 crew cab:
From the rear edge of the cab where the sheet metal meets the third brake light, measure forward 24.5 inches. Lay down a couple of passes of 2" wide masking tape. Mark that spot.
Now, measure from the left/right edges of the roof and find the left/right center.
Measure everything again to make sure you've got it centered and 24.5" from the back edge of the sheet metal.
Drill your hole.
Vacuum up all the shavings carefully. Remove the masking tape. Have a cold drink.
Go inside the cab and remove the rear dome light over the rear seats. That takes some work, but it's doable.
Once you have that out of the way, you'll see a hole in the sheet metal above you. This gives you access to a front/rear running center rib.
From the topside, push the coaxial cable back towards the rear of the cab. When you think you are close, go back inside the cab and look for the coaxial cable through that hole. Might take you a few times, and fishing it out with a bent coat hanger, finger or other tool will allow you to pull the coax down into the cab.
Pull the coax through and go back up topside and install the NMO mount. Follow all the directions that come with it. Many mounts will come with a little tube of silicone grease to put on the O ring before you put it all together. Make sure you do that so the O ring will stay in place and won't bind up with tightening down the mount.
Go back inside the cab and make sure you have all the coax there coming out of the hole over the dome light. Don't leave any up there.
I took this chance to install a rubber grommet around the hole to protect the coax. I don't remember what size the hole was.
Route the coax laterally over to the side of the truck where you pulled the gasket out. Might take a few tries, but you'll get it.
If the truck has side curtain airbags, you want to be 100% sure you route the coax between the top of the airbag device and the roof of the truck.
I got all the slack coax pulled through.
I then routed it down the back corner of the cab. I pulled the trim off (I think you need a 7mm socket to remove one screw under a cover near the seat belt upper anchor point). Route the cable down behind the rear seat to the RF deck.
If you are mounting the radio up by the front seat, you want to do all this towards the front of the rear door, and route the cable down the B pillar to the floor and then to your radio.
Once you get this far, you'll see how easy the whole process is. Carefully reassemble everything and terminate your cable at the RF Deck/Radio.
Much cheaper than the center brake light mount, and will work much better. 24 inches in from the back of the cab and centered left/right will give you an ideal ground plane for VHF use.
Let me know if you need more help. I probably have some photos somewhere if you need to see it laid out.