To order samples, just click the button in the top right of the color you wish to order. The swatch will be automatically added to your sample request order.
If you wish to remove a swatch from yoru order, you'll see that any swatches on your order will have a button. Just click that button and it will be removed from your order.
You can review your samples order at any time by clicking the shopping cart icon in the menu bar at the top.