Once you have learned your chosen profession and bought your tools, you will need to gather the appropriate reagents before you can begin crafting items. Alchemists use herbs that can be collected by herbalists. These materials can be obtained by learning a second profession, or by buying them from other players.
You can then open your crafting interface by right-clicking on your tools. You can view a complete list of all of your known recipes by clicking the "Recipes" tab.
To begin creating the desired item, first go to the Reagents tab of your bag and find the reagents that have the properties required to make the item. There can potentially be multiple different combinations of reagents that can be used to make the same potion, due to the fact that each reagent has multiple different properties. Next, drag them into the crafting interface or simply right-click on them. Then click "Boil".
Each reagent will appear at the top of one of the vials in the crafting interface, and when you begin to make a potion some of the reagents' properties will randomly appear in the bottom half of the vials. If the properties are unfavorable, you can partially re-roll them by clicking on the arrows at the top and bottom of the vials. You will only be able to move one arrow once per crafting session, but items from the Item Shop will allow you to do it additional times.
In order to successfully create a potion, you must have all of the required properties present in each vial. In the right-hand side of the crafting interface is a preview of the item that will be created if all of the current properties in the vials were to be chosen. Once you have chosen the properties, the item will be created. In order to retrieve the created item, you can simply right-click on it and it will appear in your bag.