Allow your ice cubes to sit in the freezer until they fully freeze. If you don't freeze freshly squeezed lemon juice, it can go bad in two to four days. Keeping your lemon juice frozen in ice cubes will allow you to keep the lemon juice fresh for a longer period. If you need to use your ice trays, you can transfer the lemon juice cubes to a zip lock bag.

Summary:
Place ice cubes in the freezer and allow them to freeze.