To gain skill in hfc it is important to keep the difficulty of the dish within a +/- range of 10 from your current hfc skill. That means that if you have 1 hfc you should not aim to make a dish of higher difficulty than 11. Likewise if you have 34 hfc you can gain skill from food with a difficulty of 24 but also from food with a difficulty of 44. A +/- range of 5 seems to help to get the optimal skillgain.
After placing the ingredients into your pan, etc, you can examine it and find out what you will make and what the difficulty to make it is. You can adjust the difficulty of a recipe by what you cook it in also. Cooking with an oven does not change the difficulty. A campire will raise the difficulty by 5 and a forge will raise the difficulty by 10. Another good trick is to add a leather piece or wood scrap to your pan which will raise the difficuly without using up the piece/scrap.
'''NB:''' Rumours have circulated that you will not gain any hfc skill if you leave the forge/oven/campfire. This has been proven wrong. You do not have to stand shackled to the heat source.