Enter the amount you want to convert (either in fiat currency or the coin currently being displayed), then, using the toggle button just above the Amount field, select what coin type you would like to convert to.
At the bottom of the ShapeShift display, you'll see the current conversion rate and the minimum and maximum transaction size allowed by ShapeShift.
Once you've made your selection, select Exchange. This will bring you to a confirmation screen where you can review and complete the transaction.
Please note that ShapeShift transactions may take up to 10-20 minutes to appear in the target coin's wallet screen.