How to: Backpack Rain Cover from Rain Pants

The other day it was supposed to rain and I was going to ride my bike to work. I have raingear for myself, but until the night before I didn't think about my backpack getting wet (and the clothes/stuff inside getting soaked). So I looked around my house for maybe a large plastic bag to sew into my packpack as a waterproof "lining", but I remembered that I had a pair of army issue rainpants that I would probably never wear again (not comfortable!!). I laid them out and realized that it would be a piece of cake to make a cover for my backpack with them! Here's how I did it:

First I cut the two legs apart in the crotch area (there was a seam there). Kitten help with this step is optional :)

Then I cut the seam along the inside of the leg and, one corner at a time, pinned the elastic waist to the widest part of the pant leg.

Here's one side folded and pinned. To give it a more rounded, backpack shape, sew diagonally. To have square corners sew right along the edge instead.

Once you've sewn both sides and cut off the excess you should have something like this

Here you will want to size the length to your backpack and, keeping in mind that you will need to fold the bottom around the backpack, cut off the excess.

Fold the newly cut edge and pin diagonally as shown. Sew along redlines.

You should end up with this (after cutting off excess)

Depending on the shape of your backpack you can either stop here or sew in some elastic. I choose to sew in elastic. Some other options would be making a pocket for the outside and a strap to go around the backpack for added hold (my pack is oddly shaped, so I also added a strap that snapped)

Now I'm ready for the rain!