Finely chop the basil and garlic in a food processor or high speed blender. Add the lemon juice and salt and pulse to combine.
Slowly add the olive oil, stopping to scrape down the sides if necessary. You don't have to use the full amount if it's blending up without it. You're going for a consistency similar to pesto.
Spoon the mixture into a clean ice cube tray. If you don't want to use yours, just line a tray with parchment paper and freeze the pesto in dollops on there, removing and packaging once they're solid. These last several months in a sealed container in the freezer.
Notes
You can use as much basil as you have, and change the recipe proportions accordingly.