If you have not yet received your ballot in the mail, you will need to vote in person, which you can do starting tomorrow, Tuesday, October 27. All the early voting locations are on the BOE website: https://earlyvoting.dcboe.org. Due to the pandemic, any voter in DC can vote at any in-person voting location. The website lists the approximate wait times at each location, so make sure to check before you head out to vote, and remember to wear your mask!
If you received your ballot in the mail from the Board of Elections, you can still complete it and turn it in, either by dropping it in your nearest USPS mailbox (it's postage pre-paid, so you don't even need a stamp), or by dropping it in one of the BoE's many secure drop boxes (complete list of locations is at https://dcboe.org/Voters/Where-to-Vote/Mail-Ballot-Drop-Sites).
If you decide to use your mailed ballot to vote, remember to register with the BoE's ballot tracker to get updates about the status of your ballot: https://votedc.ballottrax.net/voter/. The messages you'll get look like this:
Editor's note: I put my own ballot in the Northeast Library secure drop box and registered fro the ballot tracker app on Monday, October 5. I got the notification that my ballot had been received by the BoE on Saturday, October 10, and was accepted for counting on Monday, October 12. Dedicated W6D volunteer Frank Avery reported a similar timeframe for his ballot drop off to "accepted for counting" notice.