Twitter itself does not have the mass DM your follower function and it is not allowed to mass DM according to Twitter Rules and policies. You need to DM your followers one by one.
The process is:
Log in to twitter account >> click on the Profile button to visit your Follower list >> select you follower you want to send a message from the list of followers >> click on the envelope icon >> edit your message to send
However, with the help of some third-party automation Twitter apps, you can send message for bulk followers. TweetAttacksPro is a third-party Twitter apps that allow you to setup personalized content, and the content can be rotated into multiple personalized content using the built-in free spin tool before publishing. The followers of your account can be searched in bulk, then auto DM your followers in bulk.
This is the setting of auto dm multiple followers:
Web page: http://www.tweetattackspro.com/
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