You don't have to do it,it's done automatically.The card itself extends the memory capacity in the Gallery,which is where it stores your images,video and sound clips,as well as ring tones etc.Note:the actual memory of your phone remains the same.
I assume you want pics you take to go on the card, you need to do this;
press>menu>settings>memory settings>default memory>camera>select memory card and press save. Done! Hope this helps, do let me know...Dagman