Create a new Sequence within your Premier Pro project file.

Change the frame size to your needs.

16:9 aspect ratio - i.e. 1920 x 1080

9:16 aspect ratio - i.e. 1080 x 1920

1:1 aspect ratio - i.e. 1080 x 1080

Name your sequence for easier navigation and click OK to create the sequence and proceed.

In the Project Panel, right click and chose the Import option.

Navigate to your clip(s), select them and click Import.

Hold and drag the clip into your timeline.

Make sure to choose the Keep existing settings option when this error comes up to keep your desired aspect ratio.

Look up Auto Reframe effect within your Effects panel.

Hold and drag the effect on top of your clip.

Click the Analyze button to automatically reframe your clip to existing sequence.

You can change the Motion Tracking approach to one that suits your project best. It tells the effect how much should it be following the action within the shot.

Slower Motion - Best option for when there is very little or no camera motion

Default - Option for content where there is more motion within the shot.

Faster Motion - Use when there is a lot of motion in the footage and you want the Auto-Reframe effect to follow the action.

You can adjust the overall offset and scale of your clip after it gets reframed.

You can also overwrite automatically generated keyframes by checking the box. It will allow you to adjust the position of each keyframe to your needs.

Select your sequence in the Project panel.

Click Sequence > Auto Reframe Sequence.

In the dialogue box change the Target Aspect Ratio to the one you want to use. Set the motion tracking to the best working option for you:

Slower Motion - Best option for when there is very little or no camera motion

Default - Option for content where there is more motion within the shot.

Faster Motion - Use when there is a lot of motion in the footage and you want the Auto-Reframe effect to follow the action.

Select ‘Don’t nest clips (…)' unless:

• There are existing complex keyframes with motion that you want to retain.

• The sequence has speed and duration adjustments.

In those two cases for best outcome choose ‘Nest clips (...)’.

Click Create.

Premiere Pro will create a duplicate sequence (2) with your desired aspect ratio and new dimensions. Each clip in the new sequence has the Auto Reframe effect applied and you can adjust it within the Effects Control panel (1) (follow the ‘Auto Reframe on a single clip’ do learn how to manually adjust the effect).