Starting in the Standard workspace, double-click your footage in the Timeline panel to open the layer in its own panel, next to the Composition panel. Press Option/Alt+W to select the Roto Brush tool in the toolbar. Hold Command/Control while clicking and dragging the mouse (or moving the scroll wheel) up/down to increase/decrease the brush size (circle). Drag the Roto Brush over the main subject in your scene that you want to isolate.
Reduce the brush size and then click and drag to add more to the selection area (green brush with plus sign) — or hold Option/Alt while dragging to subtract from the selection (red brush with minus sign). Click one of the alpha boundary buttons to find an overlay style that suits you. The accuracy of your selection depends on your project. There’s no need to carefully isolate a portion of the subject that doesn’t actually have to be separated in 3D space from the background.
To clean up and refine the edges, select the Refine Edge tool (Option/Alt+W) and drag the brush around the subject to feather the edges. Toggle the Refine Edge X-ray or Alpha Overlay button to help see the accuracy of the refined edges.
Advance the small playhead in the Layer panel slightly (in 5–10 frame increments) to render forward. Wait for the green render bar to catch up to the playhead. Toggle the Alpha Boundary button and make sure the selection stays on target. If it drifts, reactivate the Roto Brush tool (Option/Alt+W) and correct the selection boundary. Gradually extend the gray trim span (direction of propagation) ahead of the playhead until eventually the whole work area is rendered. This is the most time-intensive part of the process.
When the selection satisfactorily tracks the action for the duration of the comp, click Freeze in the Layer panel to save your changes.
Switch back to the Composition panel and collapse the expanded video layer in the timeline to reduce clutter. To add some text behind the now-isolated foreground subject, duplicate the video layer (Command/Control+D) and delete the Roto Brush effect from the bottom version. Add a new text layer (Layer > New > Text) in the middle and then animate your text behind the subject in the top layer.
Now that your background is separated from the foreground, you could add a slight Gaussian Blur effect to soften it or even add the Lumetri Color effect and reduce its exposure or saturation to enhance the overall dimensionality of the composition.
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