I recently This is a quick modification I did of a drawstring bag that I got from university orientation. Adding the pencil bag saved me the hassle of digging for small objects like my ID card at the bottom of my bag. It's pretty easy and quick to make.

Supplies:
-drawstring bag
-running belt
-needle
-thread
-scissors
-needle threader (optional, but makes life a whole lot easier)

Estimated Time: 20-30 minutes

Step 1: Turn Bag Inside Out

Step 2: Cut Off the Straps of the Running Belt

Step 3: Sew in the Running Belt

A very effective way to do this is by the running stitch. First, u thread the needle and make sure you do not cut the thread from the stool yet. Then, you tie a knot on one end of the thread. You move the needle over and under the fabric multiple times. Pull the needle completely through. If you need to continue, tug on the needle so you have more thread to use and move the needle over and under again.