For people asked to sign
You received a link and need to sign something. Here is what happens.
You do not need an account
You will not be asked to register, install an app, or pay anything. Open the link you were sent, confirm it is you with the one-time code, review the document and sign.
The one-time code
Before your signature is applied you enter a short code sent to the same address or number the document was sent to. This is what records that the signature was yours rather than someone with access to your inbox.
If something is wrong with the document
Reply to the email that carried the signing link. It reaches the person who sent you the document, who can correct and resend it.