First you need to get some ½” gross grain ribbon from your local sewing supplies store – such as Spotlight here in Oz. Then you will need some of the Linelocks like you see I have sewn the gross grain to: You can buy these little guys right here in Oz, eg: http://www.tiergear.com.au/11/online-shop/cord-tension-lock @ A$0.40ea, or in the USA from eg: http://www.questoutfitters.com/Fasteners_%20Misc_Fasteners.htm#LINELOC_3_ US$0.45ea.
You sew a loop at each end of the gross grain ribbon (as shown – perhaps more neatly than this. I blame arthritis. My wife says my sewing will be plenty strong enough anyway which is the main thing! Then you pass the end loop through the tie out loop on the pack then the Linelock back through the loop on the tie down you have made. Tie a boot lace on the other loop and pass its other end through the two holes on the Linelock and you have an adjustable tie down which can be used eg to lash your Alpacka raft to the top of your pack. See above & below on my Gossamer Gear Gorilla:
The Gorilla has a pair of these yellow loops sewn into the pack on each side at front and back. Here i have used three tie downs, the middle one crossing over through the haul loop. Works well. You could also lash a Sea to Summit Ultrasil Compression Sack (http://www.theultralighthiker.com/a-tardis-folding-space/) here to carry extra food for a long hike, or etc. In this configuration the (40 litre!) Gorilla could easily haul enough supplies for two weeks’ hiking – or more!