Simple. When you're on the home screen there are two sections. The items screen (Marked in Orange) and the order pad (Marked in blue). On smaller devices, the order pad will be hidden.
The items screen is split up into Menus/Departments, Categories and Items. As you can see this site has three menus; Evening, Daytime and Bar and we're currently on the Bar Menu. Below it, all the categories are shown. As you can see we've selected the Coffee category, and then on the second half of the Items screen are all the items within the Coffee category.
1) Select one of these and they'll add straight to your order pad. However, if the item has any variants or modifiers. It will open the Item edit screen.
2) As you can see when we press Espresso there are multiple options. Once you press a variant or "choose an option" it will automatically add to the order pad. You can also add some modifications to your item or "Spice up your coffee" too. Make sure you press these first before you select a variant. You can also change the quantity and add a note.
3) As you can see, the customer wants an interesting-looking espresso. But hey, the customers are always right, amiright?
4) If you press on this item again, it will show the Item Edit screen again, where you can change the quantity and add a note.