The fastest way is to select and hold any title until a prompt appears with the option to Add to queue. Tap on Add to queue and the title will be added to the category labeled My Queue.
Another way is to select a title from the home screen, and tap the Add to Queue button located next to the play button on the titles' description screen. Afterward, you can go back to the Tubi home screen, scroll up to My Queue, and you’ll see the movie/TV show you just selected on this list.
The icon to the left of the Play button is the Add to Queue button. To add a title to your queue, tap it while it displays the plus (+) sign.
Once you've added a title to your queue, it will appear like this. To remove the title from your queue, tap it while it is displaying the minus (-) sign.