Gather all your framed photos, art collection, deco accent pieces and pictures. Stick to a color pallet to avoid visual chaos. If you are doing all black and white art, maybe paint the frames in different colors. If your art is in different colors, stick to one color frames. Measure out the wall area that you are planning to use for your collage. Measure out the same size on the floor and start laying out your pictures within that area. Play around with different configurations to get a feel for what you like best. You can also make templates of the frames out of Newspapers and tape it to the wall. Be careful leaving to much space between the frames or even putting to close together. I find that leaving about 2” between the frames tends to work best. You want the collage to feel like one art piece, not look like a wallpaper.
If you are brave and want to make the collage look even more interesting mix in 3 dimensional objects to add visual interest. You can also incorporate a clock or a mirror. Start with larger pieces in the center and work your way out. It looks great to mix in one round item with all the frames just to brake up the shapes.
Use a level if you aren't comfortable with your natural visual abilities.