DNA Sibling Test: How does it work??

Has anyone ever done a DNA sibling test?

I know you (can) order them online and do them at home.

What I’m wondering though, is how it works.. Do you take samples and send them back? Or do you get immediate results?

Any feedback would be greatly appreciated!

I’m currently in the midst of writing a novel where they have to do a DNA test to figure out whether they are really related, but I haven’t really found anything online about how to actually carry out the test… And they’re kinda expensive to just order for the fun of it 😛