I have a very broken backpack (not the one in the picture) with many exposed pockets, and since it was raining very heavily I decided to make my self a quick and easy cover.

Step 1: Materials

Backpack you want to cover
Trash Bag (generally small but large enough to cover everything)
Scissors
Marker (optional)
Safety pins or something similar (optional but highly encouraged)

Step 2: Mark Your Cut

Put the backpack inside the trash bag with the straps facing you. Mark lines on the bag where the straps will go.

If you don't want to be some punk who marks their cuts, go right on to the next step.

Some general guidelines:

I recommend starting about 4-6" above the bottom of the trash bag so that it won't eventually tear through
Always make your cut shorter than you think you need, you can always cut more

Step 3: Make Your Cut

Make said cut and you should be easily able to slide the straps through the opening.

(I apologize for the draw sting bag but it also works with regular backpacks)

Step 4: Wear It With Style!

You now have a "stylish" backpack cover that is fairly effective.

I highly recommend having someway, safety pins or something similar, to close the very large holes after putting the straps through. This will greatly increase the effectiveness of your brand new trash bag backpack cover!