If you have a kettle, fill it up and let it boil. Otherwise, boil about 6 cups of water using your preferred method.
Put the rolled oats in a bowl or large 4-cup measuring cup. Pour in about two cups of the boiled water and squeeze the juice of half a lemon into it. Stir and allow this to sit for about ten minutes.
Strain the oats through a fine-mesh sieve, then rinse thoroughly.
Add the strained oats and about 4 cups of the boiled water to your blender; this works a treat in my Vitamix, and I haven't tried in a regular blender, but I expect it would work as well, you just might need to let it go a bit longer.
Blend the oats and water at level 5 of your Vitamix (about half-power) for 30-45 seconds.
Place the fine-mesh strainer over a bowl and carefully strain the oat and water mixture through the strainer. I don't recommend using cheesecloth - I think it is too fine and it gets stuck. You may need to use a rubber spatula or other implement to kind of stir it around to help the liquid separate.
Carefully pour the strained liquid into a jar that is large enough to hold it all, seal it up, and refrigerate for later use.