To add stock firstly go to Stock management->Stock items, then select Add where you will be able to add stock along with their measurements.
For example here we will add Cheese and the measurement 'grams' and then we will select Save and return.
You can then add this Stock to different recipes by going to Recipes and compositions. Then go to Add where you can add a recipe and then select Save and edit. You will then be able to select Add again to assign stock and its respective quantity required for the selected recipe.