To add an item to your Trolley just click the orange “Add to Trolley” button. Your Trolley is just like a supermarket trolley, a place where you can store your items before you proceed to the Checkout.
From your Trolley you can remove items you no longer wish to buy and increase/decrease the quantity of individual items.
When you have added an item to your Trolley you will be able to see these items be following the links, shown inside the red circles:
From here, you will be able to adjust the quantity of the items, by editing the amount shown in the Quantity box, showing inside the red circle:
You can edit this amount from 0 - 99. Just press enter when your amount has been adjusted, or click away from the box. The page will refresh, and then you can proceed to the checkout from there.