You've probably already figured this out by now, but I was stumped on this too for a while, so I went searching online for an answer.
Am writing a response for others who go looking for the answer as well.
When you open up your equip potion table, there's a menu on the left that shows you which potion recipes you know. Clicking on each will show you the ingredients needed on the upper right hand side of the screen.
Below that is your potion toolbelt. It has three slots. One for the healing potion you get that is automatically refilled each time you camp, and two other slots. If you left-click and drag the icon for whichever potion you want from the left-side menu into an empty slot, it will show up in your potion belt (the three slots at bottom right). However, it will continue to show zero until you actually make the recipe.
Making the recipe is easy. If you have the required ingredients, you simply left-click on the potion icon you just dragged over. It's the icon on the bottom right side (your potion belt) and it will add one potion with each left click. You have to physically left click the icon on the right-hand side for every additional potion you want added (max 5).
Hope that helps someone!
