In “Dune: Part Two,” Paul hitches a ride with a sandworm by basically hailing the thing with a thumping tool and then jumping on its back with special climbing equipment called maker hooks (“maker” is another name for the sandworms, since the Fremen believe the creatures to be physical embodiments of their god). The sequence is harrowing stuff because the sandworm is moving fast enough that a misstep could spell a broken neck, but Paul manages just fine. Once the hooks are in place, they pull back the thick dorsal plates and expose the sensitive flesh beneath, forcing the worm to remain on the surface of the sand instead of diving. A sandworm will strive to keep its exposed flesh on top, to prevent the sand from irritating it, which handily prevents them from rolling and throwing their rider off.
There are also palanquins that can be put on the backs of the worms for those who can’t just jump on and hold on for dear life, which are attached once a Fremen has already established control over it. As far as climbing off of the worms, Villeneuve told Variety that he has a plan for how he would depict that potentially terrifying event:
“I knew how. I found a way. It was not dramatically necessary in ‘Dune: Part Two’ to see someone get out of the worm, but I know how to do it. And I can’t wait to put that on screen.”
Villeneuve has shown that he has no problems straying from the source material, making at least one pretty major change to “Dune” from the books, so he could have something totally different in mind, but Herbert did give a blueprint for sandworm debarking in “Children of Dune,” and it sounds just as wild as riding those gargantuan invertebrates.