1. Get some cheap metallic pens, from any old craft shop.
    
2. Have a look on the internet and look for a nice, girly pattern that you love.
    
3. Copy it down on your bag wherever you want it. Metallic pens go best with a plain, black bag. If you have a pattern which would look good anywhere, the straps are a great place to design.

Later on, you could things like logos on your bag and become a bag designer!