First, take the tube and cut to the desired length to fit your glasses. Then cut and open your tube down the long side.
Measure the width against your glasses as your tube might be a bit too wide for your glasses and cut the excess off.
Now measure the fabric against the tube. Glue your fabric to the inside of the tube’s edges and tuck in all edges gluing them to the inside. Start with the sides then with the top and bottom.
Use a small line of glue to the inside edges and press until dry. Start with the bottom then the side.
Cut the ribbon long enough to go all the way around your case, down the sides and bottom plus 1/2" extra on both ends to go into the inside of the case. Then glue the ribbon. Finally, glasses case is ready!