The short answer
Slow, swaying and easier than it looks. Moroccan camels are one-humped dromedaries, trained to carry visitors and generally calm. The awkward part is the start and finish, when the camel kneels and stands in two lurching stages. Once moving, it is a rolling walk at a person's pace, led by a guide on foot.
In more detail
You sit in a padded saddle with a handle in front. When the camel stands, it rises back legs first, tipping you forward, then front legs, so lean back and hold on. Getting down is the same in reverse. That is the moment most first-timers find alarming; after it, most people relax quickly.
On a short ride to a camp most adults are comfortable. On longer treks, two hours and more, the saddle starts to tell on the inner thighs, and our camel guide suggests padded cycling shorts for multi-day treks. Loose trousers are better than shorts either way.
It is worth doing. The silence, the scale and the slow pace across the sand are hard to get any other way. If you have hip, knee or back problems, tell the camp before you book, because the kneel-and-rise is the hard part, and ask for the 4x4 option instead. You still get the dunes.