It depends on the tests.
This is what I did for a website (from what I remember):
There was a list of instructions. It asked me to navigate around it and to write out what I saw. It also asked me to perform some actions (like click some buttons, fill out a form) and write out what I saw in response. For some of them, I would have to take a screenshot. If you came across any errors, you would write them down.
After finishing, you would submit what you have and then once it was checked over you would get paid.
That's just one example. There's a bunch of tests, but these are the ones I liked to do since they didn't take up as much time as some of the others that were more in-depth.
From the site I used, I remember having to do an entry test (which is like the one I just described) to become a user.