If there is a big contrast between the solid color part of the background and the main body, directly use the magic wand or color model to select the background and fill it with white.

If the background is complex, tick off the main body with a pen, test the layer and fill the background with white.

Taking the above picture as an example, it is most appropriate to dig this picture with a pen. Method:

Final effect

Draw the outline of the shoes with pen and ink.

CTRL+Enter converts the path into a selection.

Go to the layer (CTRL+J), create a new layer (CTRL+SHIFT+ALT+N), fill the foreground color white (ALT+DEL), move this layer down one layer (CTRL+[), and then add a shadow to the shoes. Done.