You will need a piece of fabric that is large enough to house your Pi Zero case and battery pack alongside each other with enough spare to hem the pocket all the way round. If you have access to a sewing machine, then this section of the project will be much quicker, otherwise you will need to do the stitching by hand.

The piece of fabric I am using is 18 x 22 cm allowing for a 1 cm hem all around. Fold and pin the 1 cm hem, and then either stitch it by hand or run it through a sewing machine to secure your hem. When you have finished, remove the pins.

You need to then turn your garment inside out and decide where you are going to position your hidden pocket. As I am using a t-shirt for my garment, I am going to position my pocket just inside at the bottom side of the garment, wrapping around the front and back so that it sits across the wearer's hip. Pin your pocket in place, and then stitch it along the bottom, left, and right sides, leaving the top open. Make sure that you stitch this nice and firmly as it has to hold the weight of your battery pack. The picture here shows you my pocket after being stitched in place. When you have finished, this you can remove the pins, and turn your garment the correct way round again: