I start with one picture for the middle. Unless I use one picture as the background. Usually I don't fiddle with opacity, but that is certainly very effective, especially in fading the background for a base. As I bring in each photo, I adjust the size, sometimes directly, sometimes I use the photoshop tools to scale them just right. The further along I get in the project, the more I am apt to use the scale tool, and then the mover tool, to fit the photo into a desired spot. Magnifying sections lets me see how well I am doing, as often the photos have to butt against others, but sometimes I overlap for a design effect. I haven't experimented with putting layers in front or back of one another...have to get into that. Also want to get into selection features so I can make backgrounds disappear if needed. But none of that has been necessary so far and I have been very pleased with just jigsaw puzzling around and making a composite of favorite and topical photos, usually combined with some type in appropriate colors. You have to remember to sample the color and adjust the font and size, and I keep each word separate, so I can move them individually if there are only a few words. That way, I have control over how the type is placed.
All in all, time flies when working on one of these projects, which really is a symbiotic process...a way to connect with the person I am making the montage for, the event such as a birthday, and the walk through the photo albums of time to create a world of special meaning. TG for photoshop and all the fine art tools it has to offer. Sampling a color helps with a quick airbrush to remove some errant colors in a snapshot, or selects just the right color of type to blend with colors occurring in photos. As with some watercolor collages I have created, you can just airbrush around some pictures, or draw with the brush, to continue a scene where a picture leaves off.
New ideas just seem to grow, from one project to another. Had a bit of a printing problem till it was revealed that the 32 bit did not gel, nor the tiff. Had to print at 16 bit, and pdf. Stay tuned.