Decide and cut your cardboard or poster board to desired collage size. I would recommend using a standard framing size like 8 X 10, 9 X 12, 11 X 14, or 18 X 24 so that you can frame your work when your finished. Collect all the tools and materials you will be using for your FABULOUS collage.
Decide on a color theme for your collage. Using the materials you have, create a "palette" of colors you will use for the piece. Try to use the colors that frequent the colors in your photos. For example, if most of your photos are pictures of a wedding, use similar colors of paper and odds and ends that match the colors of the wedding party. If you have BW photos, the color palette you use is easier to choose.
Cover the poster board (or card board) by gluing down decorative papers, construction paper, or newspapers without any attempt to put pictures on it yet. Pretend you are doing a "work of ART" with the materials you have collected to use. Here are some examples for you to use as a springboard for ideas:
*cover the area with vertical stripes of different colored paper.
*cover the area with squares of colored paper, found paper, magazine cutouts, or of fabric.
*cover the background area using one sheet of decorative paper.
All of these are examples of how you can handle the background of your photo collage.
After you are happy with your background (and you feel like you could put it in a frame and be happy with it as is) then work on cutting, cropping, and embellishing the photos you will use for the collage. You can theme the cuts on the photos by using the same decorative scissors to cut them, you can add a paper frame around each one to embellish them, or you can leave them the way they are to create a natural picture look. Any way you chose to adorn your pictures is fine, as long as you don't use TOO MANY styles together. It doesn't look good when photo cropping and adorning compete for the viewers attention.
Begin by placing photos on the background as a "Dry Run" placing them in different places to see how they fit. Arrange them so that the more interesting parts of your background show and the less attractive parts are covered. Don't commit to gluing them until you are completely satisfied that the pictures are in the right place. When you are satisfied, glue them down.
Now, embellish your photo collage with your found objects, odds and ends, and other materials you collected for your Art piece. For example, if you had a thin strand of lace you intended on using, glue it down so it passes over a part of a picture that is not in the focal point so that it creates a "layered" effect on your work. Another great technique is to paint over the whole collage with a Polycrylic or Hodge Podge sealant and then after it dries, add a tint of paint or stain to antique the whole work. Truly, the "sky is the limit" when coming up with decorative ways to create you own personal photo collage.
When finished, attach wall hanging hardware by gluing it on the back, or put your collage in a frame. Another great idea is to put it on an end table or coffee table and cover it with glass so it can be a conversation piece when guests are visiting.