Crafting items takes time and skill.For every eight hours spent crafting you create 25gp value. So, for example, a Healing potion worth 50gp would take 16 hours to complete. A Greater Healing Potion worth 100gp would take 32 hours. Both would require proficiency with a Herbalism kid or Alchemist’s supplies and herbs found using the survival skill or supply's bought at a store. Hours spent do not need to be consecutive, they can be added up over time and done before or after rests or during downtime.
-Ryan’s personal rules- At the completion of appropriate hours, you will roll a d20. On a roll of 1 or 2 you will fail your crafting. On a roll of 19 or 20 you will have a critical success, this may mean you created extra of what you were working on, or a version a bit better. Proficiency and appropriate tools will give you an advantage on this roll allowing you to roll twice. For example, proficiency with cook’s utensils while owning cook’s utensils.