Decide how much foam core will show around the picture. Measure the picture and add the number of inches to the extra inches needed for the mat. For example, a two inch border will require an extra four inches on the length and width.
Cut the foam core to the size you measured in the step above. Use a ruler to guide the utility knife in a straight line.
Measure the border and mark with a pencil. Using a ruler to keep your lines straight, either mark the corners or draw a full outline of where your print will go. Double check your measurements by placing the photograph in your marks to see if it fits with the proper border.
Prepare an area in which to use the spray adhesive. The adhesive can travel through the air, so use it outside or in a well-ventilated area. If using inside, spray over newspaper or a cardboard box so the adhesive doesn't stick to furniture or the floor.
Spray the adhesive to the back of the print. Apply liberally, but do not soak the print.
Align the print to the marks you made on the foam core. Press one side down first and, checking to make sure the photo doesn't slide around, slowly work toward the opposite side by rubbing the print with a clean hand or arm.
Erase any visible pencil marks on the foam core.