Activate either The Thief or The Lover Guardian Stones to improve the skill faster. The Thief stone increases the Shadow skills at a faster rate (20%). The Lover stone increases all skill 15% faster. Then just keep making potions. It would also probably
be a good idea to learn as many properties for the ingredients as you can so you can make potions and poisons with multiple effects. You can also seek out trainers and skill books.