Measure your gps and add 2 inches on each side and make it 3 1/2 times as long. Cut out 1 piece each of main fabric, lining and interfacing. Then cut out a piece of fabric for the cord pocket and a lining piece.
Here I've given you the measurements for a garmin.
Here's the pocket.
This is for the tab.
Iron on the fusible fleece to the main fabric.
Cut a piece of fabric for the pocket top binding.
Sew the strip to the pocket right sides together.
Turn under the raw edge, pin, and sew the binding to the pocket.
Sandwich the pocket in between the right sides together of the bag with the pocket on the right side of the bag and the lining on top.
Make the tab. Round one end and sew right sides together.
Place the tab at the top and pin in place.
Sew together and leave a spot for turning. Turn.
This is what you will have so far. The next step is to sew the bottom of the pocket on to the bag and then topstitch all around.