1. Greek yogurt is a thick, creamy, strained yogurt style. It comes in a variety of flavors, including plain. In the raspberry Meyer lemon smoothie recipe, I've called for raspberry yogurt, which adds more raspberry flavor and also some sweetness (since the rest of the ingredients are very tangy). If you're working with plain yogurt, you can add a spoonful of raspberry jam. You can also choose your fat content, from skim to whole milk. This smoothie will work regardless of your choice — just know that in addition to providing creaminess, fat carries the flavor of other ingredients. If you like, you can substitute Icelandic skyr.
2. Meyer lemon is a beautifully floral, less-sour hybrid of lemon and mandarin orange. They tend to be quite juicy and well-behaved, so the ¼ cup of freshly squeezed juice called for in this recipe should come from one fruit. You can use the measurement approximately.
3. Flax seed (also called linseed) has a mild, nutty flavor and packs tons of protein, fiber, and good fats. I like to buy it ground and store it in the freezer along with my other superfoods and frozen fruits. I've added it to this recipe for a little extra bump of nutritional value, but it's optional.
4. If you're using a regular blender, you will need to add at least ¼ cup more liquid. I recommend cold water.
5. Smoothies are at their best shortly after blending. But if you need to make yours in advance, you've got a couple of options. Store it in a regular cup in the freezer for up to an hour or in an insulated, lidded cup in the fridge overnight.

More recommended flavor combinations for Greek yogurt smoothies