Assemble the Images
Cut the main image from its background by selecting around the subject using your favorite selection tool. Feather the selection using Select > Feather and invert it using Select > Inverse. Press Delete to remove it. When the subject is isolated, drag its layer and drop it into your working image where it will appear as a new layer. Size the subject using the sizing handles and move it into position.
Objects, such as portraits that are cut off at the neck, look best when anchored in the image. To do this, place the face on an object (for example, a table or inside a window frame) or place it at the bottom corner of the image (see Figure 5). Objects that have no visual support are harder to work with because they look like they are floating.
Figure 5 An object, or person cut off at the neck, looks best when anchored on the page—placing it in the bottom corner of the image is often a good choice.
Select the other images to use. Choose whether to remove them from their backgrounds before adding them to the collage—as you did with the main subject—or drag and drop the entire image layer into the project and work with it there. However, ensure that each object has its own layer.
To size an object on a layer, Control+click on the layer thumbnail to select it (Command+click on the Mac), and click the Move tool. Hold the Shift key as you drag on one of the corner handles to size the object in proportion. Unless you really want to distort the object, it’s best to keep the ratio of its width to height consistent.
When an object points the wrong way, flip it to face in the other direction by Control clicking on the layer thumbnail (Command+clicking on the Mac). Click the Move tool and choose Edit > Transform > Flip Horizontal. Avoid doing this for recognizable people, buildings, or anything with readable text because you’ll be creating a mirror image of the original.