If you have a raspberry plant in your yard, you can simply pick the leaves from the plant. You may also be able to find them at your local farmer's market. You can pick the leaves any time of year. Make sure to rinse the leaves thoroughly in fresh water. Leaves can have residue from pesticides and other chemicals in the air. Drain the water off. Pat the leaves dry with a towel. Spread out the leaves on a clean, flat surface, and allow them to dry. Once they are dry, you can store them in an airtight container. Instead of finding and washing raspberry leaves yourself, you can simply by red raspberry leaf tea at a health food store. Boil a cup of water to brew your tea. You can prepare it as you would any loose-leaf tea. You need about a tablespoon or a little less per cup of water. You can use any type of tea strainer you wish to brew it, including the mesh ball strainers. Let the tea steep for about 5 minutes. Once the tea is done steeping, drink as you would any tea. You can add sweetener if you prefer.  Some suggest letting it steep for 10 minutes is better than 5 minutes.  Try chilling the tea and adding a squeeze of orange juice.

Summary:
Find fresh red raspberry leaves. Wash the leaves. Dry the leaves. Boil water. Measure out your red raspberry leaf. Steep the tea.