Step 1: Collect Valentine's/love theme-cards and select one of them to use as a template. You may want to use a smaller rectangle - choosing a larger one may mean your template won't fit on the smaller cards. Unless you want to create several different sizes of triangles.
Step 2: Make your template. Tear the card in half along the crease mark. Like this:
Then, using the back half of the card (the one with the main message), fold it in half. Like this:
Next, using a ruler, draw a diagonal line from one corner to the other. Like this:
Cut along the diagonal, unfold, and...voila! your template.
Step 3: Cut out your pennant pieces. Using your template trace around the colorful front of the card and cut out. At this point, you may want to write on the back who the card was from, who the card was for, and the year. Also, you may want to play with the order of the pieces to get a pleasing pattern.
Step 4: String your pennant pieces together. Punch two holes at the top and thread string, ribbon or raffia through the holes. Holes small in diameter are better than larger ones unless you are using a thick material to string the pennants - you want to be able to slide the pennants along and adjust their position easily.
Step 5: Hang up on the day of the celebration and enjoy. Tuck away for the next Valentine's Day and add to it as you collect more cards.