Often when you are backpacking, you need to strap items to the outside of your pack. Things like tents, sleeping bags, sleeping pads and walking sticks are usually long, round, and too bulky to put inside your pack. Packs normally have cute little attachment points on them. These normally are designed to work with packing straps, but packing straps are often expensive, the wrong length, or hard to find. Hence this simple system that uses light rope or heavy string or cord (I'll refer to this as cord).
My family recently field-tested this simple system by hiking the Chilkoot trail (over a mountain pass between Skagway Alaska and Canada). This system was simple, very stable, and worked easily to access the strapped on stuff, and to reattach it repeatedly.
Step 1: Cutting the Cord
Firstly, often these cord materials don't cut very well at all. Even if they do cut, they become frayed at the end. The best solution I have found is to use a lighter's flame to both cut and finish the line. This works well with all synthetic cord that I know of.
Step 2: Thread the Cord Through the Backpack's Attachments.
Note that this attachment was designed for a strap, but the cord works fine. I find it useful to use a screwdriver or a knife to push the cord through with. This is a task that doesn' t normally need to be redone in the field, so the fact that it is fiddly isn't a problem.
Step 3: Tie a Loop in One End
The simple loop knot is very easy to tie. Tie it onto one end of the cord. (Note that the cord doesn't need to be cut to length yet. In fact it is better if its long so you can use your gear (tent, pad, etc.) to determine the ideal length.)
Step 4: Cut the Cord to Length.
Now it is time to cut the cord to length. There should be about 4" of cord hanging loose when the cord is lightly taut holding the equipment.
Thread the free end of the cord through the loop on the other end of the cord.
Cinch it down! With this system you can tighten the cord surprisingly tight. This is really good, as it holds your items secure.
Step 5: Threading the Loop
Once the cord is cinched, with your left hand (for a right-handed person) pinch the point where the free end goes through the loop. This holds the cord secure so that you don't loose your cinching. Using your right index finger, push a loop through, just like tying your shoes.
Step 6: Finishing
Lastly, before releasing your left hand, pull the loop tight with your right.
In the field, of course, you only need to pull on the loose end to open the knot. Do not take the cord out of the backpack's attachments in the field! You only need to redo the last three steps to re-attach your items in the field.
Having your items attached to your pack in a good, stable manner is a heck of a lot nicer than fighting a pack that has bits falling off while you hike. I know, I've tried that.