I have no idea how it’s done in WI, but here in MA:

1. You register to vote, including providing your signature on a registration card.
2. You request a mail-in ballot
3. when you receive the ballot you fill it out
4. You place the ballot in a blank envelope that came with the ballot.
5. You place THAT envelope inside a second one that’s already addressed back to city hall, and includes your name/address on the outside (and I think a barcode). That envelope also has to be signed by you or the ballot won’t be accepted.

My understanding is that when city hall receives it that it goes through the following process:

1. They confirm you actually requested a mail-in ballot and that one was in fact mailed to you.
2. The signature on the envelope is at least a rough match of the one in your voter registration.
3. The voter rolls are checked to confirm they have not already received a ballot from you.
4. The inner envelope is removed and confirmed to be sealed.
5. The voter rolls are updated to indicate your vote has been received.
6. The outer envelope is thrown away.
7. The inner envelope, still sealed, is added to the pile of all other ballots that have been received.

On Election Day those envelopes are opened and the ballots are counted.

If at any point in the process a discrepancy occurs then a formal investigation is launched. This includes receiving more than one ballot from an individual, somebody showing up to vote in person after a mail-in ballot was received for them , etc.