A great option to sew a cosmetic bag. It is notable that you are building up fabric on the edges of a zipper, so it carefully comes into the corners of the bag.
It's easy! I show everything in detail in this video tutorial.
Fabric sizes from video:

• 2 outside parts 25cm by 19cm;
• 2 inside parts 25 cm by 19 cm;
• 2 details for pocket 19cm by 12.5 cm;
• a piece of cloth to process zipper 12cm 9 cm.

You will also need:

• zipper not less than 22cm long, can be more;
• interlining to strengthen the outer fabric, 2 pieces 25cm by 19cm.