Choose Video Effects > Transform.

Drag the Auto Reframe effect to the clip you want to reframe. 

Fine-tune the Auto Reframe effect by choosing a suitable motion preset from the Effect Controls panel. While applying the effect, Premiere Pro generates motion keyframes that follow the action in your content. You can choose one of the following options:

Appropriate for when there is very little or no camera motion (such as talking head interviews). When this control is used, results are almost static and the clip has very few keyframes.

Appropriate for most content. Here, the Auto Reframe effect follows the action, however the results may not be great if the action is very fast.

Appropriate for when there is a lot of motion in the footage and you want the Auto-Reframe effect to follow the action (such as in action sports or skateboarding videos). Here, Auto Reframe ensures the moving object is always in the frame, and adds a lot of keyframes to your clip.

(Optional) After the effect is applied, play back the clip. If needed, you can copy and edit the keyframes in the Effect Controls panel to fine-tune the results.

Select Auto Reframe Sequence. Select the sequence in the Project panel.

• Click Sequence > Auto Reframe Sequence
• Right-click the sequence in the Project panel and select Auto Reframe Sequence from the context menu.

In the dialog box that opens, select the new aspect ratio from the available presets or specify a custom aspect ratio.

Premiere Pro creates a duplicate sequence with the correct dimensions for the new aspect ratio. Every clip in the new sequence has the Auto Reframe effect applied. This duplicate sequence is saved in the Auto Reframe Sequences folder in the root of the Project panel.

You can see the analysis progress of Auto Reframe on the progress bar on the bottom right of the screen as it works in the background.

Fine-tune the Auto Reframe effect by choosing a suitable motion preset from the Motion Preset drop down list. You can choose one of the following options: Slower Motion, Default, or Faster Motion.

Appropriate for when there is very little or no camera motion (such as talking head interviews). When this control is used, results are almost static and the clip requires very few keyframes.

Appropriate for most content. Here, the Auto Reframe effect follows the action, however the results may not be great if the action is very fast.

Appropriate for when there is a lot of motion in the footage and you want the Auto-Reframe effect to follow the action (such as in action sports or skateboarding videos). Here, Auto Reframe ensures the moving object is always in the frame, and adds a lot of keyframes to your clip.

(Optional) To preserve your original motion adjustments with your reframed sequence, apply clip nesting.

If you choose to nest clips, Premiere Pro places all of your video clips into a nest and uses the original cut points and track layering, leaving your graphics and audio unaffected. 

Select nesting if you have:

• Existing complex keyframes with motion that you want to retain
• Sequences that have speed and duration adjustments

If you have a complex sequence with multiple points of interest or rapid movement, you may need to fine-tune the keyframes in the sequence after reframing.