This post comes from the 10th edition of the Boy Scout Handbook. (my book that I used as a scout)

The pants pack is a very simple day pack to make while hiking.

1. Close the snap and zipper of the pants
2. Using 1 foot of cord, tie off the cuff of the left leg. bend the pant leg at the knee. with the extra cord, tie the cuff to the first left belt loop.
3. Repeat step 2 for the right leg.
4. Using about 3 feet of cord, thread through all belt loops. You will use this to cinch the opening of your newly formed pack.
5. Load with your gear and lunch and close the top with the drawstring
6. Slip your arms through the pant legs and enjoy your hike.