"It's the end of the world, man. Who cares where I keep my beans?"
I mean, where else are you going to keep all your loot?
Although this is not strictly a tutorial, I thought I should give everyone a little example of a free backpack in use and tell you what I did to make it look legit.
Apologies in advance that I never took any pics of the work in progress (I may remedy this soon) but at the risk of telling you how to suck eggs, I'll detail the steps I took after cleaning and preparing the bare metal model.
Step 1: Cut two thin strips of card.
I used a standard index card but the thickness is entirely up to you. The width is really up to you too and will be dictated by your preference and the back pack you have. I chose to make them quite thin for this model as novelty stuffed animal backpacks tend to have very basic, nylon straps barely worthy of the name.
Step 2: Carefully superglue the ends of the strips to the back of the model where they would normally attach to the backpack.
Now this is where some WIP pics would have been handy, eh? Sorry! Anyway, I'll do what I can to make this clear.
I began at the bottom, on the lower sections of strap and worked my way up. Once the ends of the strips were fixed in place, I carefully "painted" superglue onto the strip's inside surface (the one that will be in contact with the miniature) using a small triangle of plasticard - it doesn't matter what you use though - then pressed it into position until it stopped just under the arm (fig.1). Once that had set, I trimmed off the excess card, leaving a nice clean strap. As a final step I painted a fine amount of superglue over the strap to seal the surface of the card but PVA will work just as well (just not as quickly). Repeat this step for upper sections of strap and the other side.
You will see in fig.2 that I trimmed the excess card off at a steep angle where the straps disappear under the armpits.
Step 3: Stick on the backpack.
Err...just stick on the backpack.
And that's it! This is by no means the definitive method but it is the simplest and least fiddly approach for consistent and realistic results. However, if you are experienced or feeling brave, I would definitely encourage you to sculpt the straps on using your choice of putty as this will give you far more options for details like buckles, padding or whatever else.