TY the Tasmanian Tiger

This is pretty self explanatory. Simply scroll to the Rang Store on the main menu, and you will have the option or purchasing various rangs with collected opals, or with real cash. The rangs vary in prices, and many are expensive and require lots of grinding. You should be able to buy at least 4 of them just by completing the levels while you attempt to beat all 3 zones. The last one or two may require you to grind for after completing the game, so simply start up any level and grind, as stated in the Opal Loaded achievement. Alternatively, you could buy each rang in the shop whenever you wish for $1.49 each. If you get towards the end of the game and don't feel like grinding any more, you can buy the remaining rangs.