If you’ve decided to buy a specific item, you just have to left-click the little gold bag next to the item in the middle window of the item shop (A). To be able to buy the item, the gold bag item has to be displayed in colour - if it is greyed out, that means that you have insufficient diamonds to buy this item and would have to top up diamonds to buy it (go to "Topping up Diamonds").

As soon as you click the Icon, a password prompt (B) will open, which asks you to enter your secondary password to continue with the transaction. Once you have done that and confirmed this by pressing the "Accept" button (C), the item will instantly be put into your Item Shop Bag (go to #8).

Important: You only have to enter your secondary password the first item you buy in that play-session - If you buy another item, it will be instantly bought upon clicking the gold bag icon! However, reclogging reactivates the password prompt.