Leather Shell Sling Bag
You can have this acrylic pattern from here:
http://www.leathercraftpattern.com/leather-acrylic-pattern-shell-bag-template-A-130
The finished size is about: 6.5" x 6.9" x 3.0" (16.5cm x 17.5cm x 7.5cm).
Advice:
Hardware: 831 snap x1.
Leather: We advice 2.0mm (5oz) leather, size 39cm x 51cm, 3mm leather cord 2m.
Tools: If you are a new leather player, try to considerate to have tool kit here: http://www.leathercraftpattern.com/leather-tools-leathercraft-tools/leather-tool-kit/leathercraft-tools-leather-craft-patterns-leather-art-leather-carving-tool