Step 1: Steam (or blanche) frozen green beans just until tender and no longer frozen.
They should have a lovely bright green color and have a good snap to them.
Step 2: Add to blender and blend. You probably won't need any additional water, but if you do, make sure you've reserved that cooking water and use that instead of regular tap water.
This is how much a regular bag of frozen green beans will make once pureed.
Step 3: Spoon into trays and freeze! Done! Yay! If you want to strain, feel free, but you will lose some volume. If you want to make them super smooth, I'd go ahead and strain, then re-blend them. But I don't.