Step 1: You will need:
* Ice cube trays
* Oil (that you won't mind the taste of in your smoothie, like coconut or something neutral like grapeseed)
* A freezer
* A narrow spoon and/or a CLEAN flavor injector
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.
Step 3: Fill your trays
If you are using the syringe, swirl it around a bit as you fill the cube to make sure it fills evenly with no air bubbles. You can try to smooth out the top if you like, but since you are going to pulverize the end result anyway, as long as there are no giant peaks that would stick to something as they freeze it seems pointless to worry about whether it is smooth or not.
You can certainly fill the cubes with a narrow spoon if you don't have a syringe! In fact, you will likely need to use a spoon to do the last few cubes when you are running out of yogurt at the end.
I do recommend the syringe if you can. As you can see in the second picture, the two full trays had very little mess outside of the cubes, but the partial tray is noticeably sloppier (that is where I had to switch to the spoon).
Step 4: Stack and freeze
I set these up early afternoon on a Sunday (2PM maybe) and went to use them first time on Monday morning around 7AM. They were solid, but I would say Tuesday morning they were really hard and easier to work with. If you are using a lower fat yogurt they may solidify faster than that. Monday's smoothie turned out fine though.
Step 5: Enjoy!
Pop a few in your blender in place of water ice cubes and enjoy a creamy boost of protein in your smoothie drink!
Hope this has helped you streamline your process!