Collages, or photomontages, are different from most photographic pursuits.
The creation of a body of work is often spread out over time.
Yes, the press of a shutter is quick.
Followed, perhaps, by knowing you got something.
But, photography is often like cross-country skiing.
Doing a collage can be more concentrated and intense—like downhill skiing or snowboarding.
You're there at the creation, the click, and the creation, the object, the print.
In addition, collage allows a photographer to return to pre-Renaissance conventions of art.
Before the development of perspective, a painting could incorporate many views, most often a narrative, of a subject.
With collage, you can discard the single-point perspective of the camera, and can play more easily with sequence, narrative, and cubism.
There are lots of choices to be made when doing a collage or photomontage.
1) Select a field/background.
You can use Photoshop to create collages on photographic paper.
However, you're not limited to a smooth or matte piece of white photograph paper.
The substrate for your collage can be just about anything.
The background can be a color, a photograph, drawing, scan of some fabric with a pattern, or ?
2) Two dimensions, or three?
Photographers deal with three dimensions, but on a two-dimensional surface.
Have fun with three, physical, dimensions, if you wish.
3) Level of craft required
Do you need/want perfection, or are "ragged edges" more appropriate?
Photographers, more often than artists using pigments, may have a perfectionist streak.
Collage can make photography more like the finger-painting experience, if you like.
The materials you use can be anything and everything, as long as they don't rot.
For example, put a mirror in the middle, so the viewer's image is part of the collage.
You can mix in non-photographic materials, such as fabrics, handmade papers, clippings from newspapers and magazines, Velcro, items found in the gutter (hubcaps, dead umbrella parts, etc.), string, and so forth.
You can add pigments, such as spray paint (use a stencil?) or pastels.
5) Play with the formal elements
These elements include lighting, composition, color, tone, depth, volume, and so forth.
For example, your collage could be "illuminated" by light from a certain direction.
Or, you could create depth by using reds, which advance, and blues, which recede.
6) Be cinematic/narrative.
The left edge could start at 10 P.M. and end up at 5 A.M. on the right edge.
7) Play with cubism.
Take pictures of someone from 3 perspectives.
If doing a portrait, the frame/border could be the person's favorite foods, activities, clothes, and so forth.
How about some dried pasta as a border?
Collage A famous essay by Clement Greenberg, an art critic
Go to Make a Collage.
Learn Photomontage from a Pro Steve Caplin