Step 2: Oil your cube trays
Take a bit of your oil on your clean finger (or, in the case of something like grapeseed oil, you could start with a small drop in each cube). Smear a thin layer on all the sides and the bottom of each cube section, going all the way to the top. It does not need to be so thick that the oil puddles on the bottom, but you do want to make sure you get adequate coverage.
This is why you want an edible oil - you are likely going to end up with some of it in your smoothie. I have found coconut oil is quite pleasant, even with berries. but anything you don't find offensive really should be fine.