It’s super simple, pick as many of each type / height of tree that you want and add it to the pallet. Keep going until the pallet at the bottom of the screen turns green, at which point you can add the pallet to your basket. Be careful not to overload the pallet though, or it won’t let you add it to the basket.
You can then add multiple, mixed pallets, to your basket either using the same mix by simply repeatedly clicking ‘ADD TO BASKET’ or you can tweak the mix as required. To see the summary of what is in your basket, click ‘BASKET’ at the top of the screen.