Well first you need to understand what a spell is. A spell is basically a liquid spell. So a tea, lotion, perfume, bath, even a soup, can be called a potion. Also remember not all potions are to be consumed.
Like with a spell, figure out what you want it to do [luck, protection, wealth] then choose ingredients with these properties.
Let's say you choose a spell to cure a cold. Take a quarter of a lemon, squeeze out the juice, then take a teaspoon of honey [or more depending on how sweet you want it] then fill the cup with boiling water. As you stir it, charge it with energy and visualize feeling better. [You could also cup your hands around it and charge it] finally, visualize yourself healing as you slowly sip.
That's an old recipe but you get the idea how you would make a potion. [Tea specifically] one final point, when making a bath [throw herbs/sea salts into the water] some feel you should fill the tub with pure hot water and let the herbs steep like a tea [and you get in once it cools down] I don't so I think it's personal preference.
So... If I were to say make a luck potion, I'd get different items that portre lucky qualitys and mix then together with the purpose at hand? As well as cast a small charm over the potion to, how do I put this, enhance? The potions quality?
Well first what type of potion did you want? I say this because some potions have ingredients which are poisonous or inedible [my first potion included gold flakes and a smooth river stone. Instead of drinking, you were to carry it in a bottle] so for argument, you want to make a tea.
Take a pinch of allspice, a pinch of cinnamon, and a pinch of nutmeg and place it in cheesecloth and tie the corners together. [This would make a teabag] as the water boils, hold the teabag and charge it with energy. As you pour the water into the cup over the bag, visualize on your desired outcome. Let it steep [this will make the water taste like the tea] while this is happening you can do a few things, you could charge more energy into the tea, you could say a luck chant, or you could visualize the desired outcome. When the tea has steeped [3-5 minutes] slowly sip the tea, be aware of everything.
That's basically it. Like with any spell, you could add a chant, you could swap ingredients, you could charge each item individually or together, whatever feels right for you. You could also use this in the bath, the steps are the same, but the steeping might take longer, and instead of drinking the bath, you would soak and breathe in the aroma. Or you could bottle the water once it cooled and wear it like a charm.
