Coarsely grind a teaspoon or two of oatmeal. Moisten with a liquid such as honey, aloe, distilled water, spring water, or milk.
To use as a mild exfoliating cleanser: use it immediately and make fresh each time. Apply a small amount to damp skin, gently massage the skin, leave it on for a minute, and rinse well.
To use as a non-exfoliating cleanser: moisten with liquids. Wait several minutes until the oatmeal is softened (when oatmeal gets mushy, it no longer exfoliates), and then use.
Store excess in the fridge, and use within a few days (you may need to add more liquid each time you use it). (If you do not wish your oatmeal cleanser to exfoliate, do not make this cleanser with cow or goat milk, since they are mild exfoliants too. However you can make this version with cow or goat milk if you wish–it just won’t be a physical exfoliant but will still exfoliate using the milk’s natural lactic acid)
Some people prefer to cook the oatmeal to soften it faster. If you cook it I recommend making it with water or milk, and then adding other liquids (such as honey or aloe) afterwards.