How to set/change the order of who signs

When you're setting up a document for someone to sign, whether from the Sign or Send tab or Create a Template tab:

Click +Signer to add signers to the document.
When you add signers, the Set signer order option will appear with a toggle next to it.
Once you toggle Set signer order to on, you'll notice that a grid of dots appears to the left of the signer role.
You can click and drag the grid of dots to rearrange the order of signers.

When Signer 1 has signed the document, Signer 2 will be prompted via e-mail that it's his or her turn to sign, and so on.