LOL OK, you want the skinny step by step.
1. Cut the poster board to 1/2" less wide and high than the mat. So for a 16x20 mat, 15.5x19.5 on the poster board. This will be your backing.
2. Get the mat centered over the photo. Can be tricky. Place the photo on a table, face-up. Position the mat over top of it so there is no white area on the border of the photo showing, and the photo is centered. Slide both mat and photo so that they're hanging off the edge of the table. Tape the photo to the mat at the two corners hanging off the table. Your photo is now positioned securely.
3. Now flip the mat/photo upside down. Tape all the way around the edge where the photo meets the mat, making sure the photo stays flat and doesn't move (the tape on the corners helps you here until you're well underway)
4. Spray mount the back of your posterboard according to the instructions. Then press it on to the back of the photo/mat, making sure it is smooth, especially around the corners. I use a credit card in even strokes for this.
5. Voila! See? Wasn't that easy?
It gets faster and easier with practice. But don't try to rush it, you'll end up with a ruined mat, a ruined photo, or both.
T