To DM on Tumblr, you can either use the direct messaging feature on the website, or you can use a third-party app like TweetDeck. To use the direct messaging feature on Tumblr, go to the person's profile and click on the "message" button. To use a third-party app, open the app and sign in with your Tumblr account. Then, search for the person you want to message, and click on their name.