If starting with a whole melon, wipe it clean with a damp paper towel. Place a dishtowel under your cutting board to keep it steady. Place the melon on the cutting board. Use a long, sharp knife to cut the melon in half, from stem to root end. Scoop out the membrane and seeds with a spoon and discard them.
Place a melon half face down and cut it vertically into 1.5-inch wide slices. Take one of the slices and use a small knife to cut off the skin and a bit of the dark flesh of the fruit, which you will discard. Cut the remaining slice into chunks, until you have 2/3 cup.
Place ingredients into a blender, cover, and mix on high speed until smooth.
You can use regular or unsweetened almond milk/cashew milk. I haven't tried this with oat milk.Dairy milk also would work, if you don't need this to be a vegan smoothie.