How to make a before and after image

Matching the two halves is the whole job

If one photo is larger, closer or brighter than the other, the comparison stops being about the change and starts being about the photography. Matching size and shape first is what makes the difference believable.

Shoot the after from the same spot as the before whenever you can. It is the one thing that cannot be fixed afterwards, and it is what separates a convincing comparison from one that invites doubt.

Label them. Before on the left and after on the right is the convention in most places, but conventions are not universal and two small words remove all ambiguity.

Lining up the two halves

The comparison only works if the two pictures are framed alike: same distance, same angle, same light. Crop both to the same shape first, so the halves line up when they are joined, and use a small gap of 8 to 16 pixels to separate them.

Put the before first, because the picture added first appears on the left, or on top for a stacked layout. If you want a label, join the pictures first and then use Text & stickers on the result to put a caption along the top or the bottom.