First you need to unlock enchanting by doing the first experiment with your aunt. Open your Inventory with "I", theres two types of racial consumables, drinks and food. If you defeated some enemies and stole their stuff you should have at least some of those. Look for the food, like "Gators Gumbo", "Boiled Eggs" etc. If not you will have to buy it or randomly find it in hostile zones.
Select the food and click enchant (standard key "5") It should open a menu like this:
If you have far less options to select, you propably have chosen a drink instead of food. In that case click back and try to find a food item instead. To increase your dick size you select the penis icon (3rd rod, 2nd item). This should open more options in the right box. There you can select size (first row, 3rd item). With the buttons in the middle you can select how much you want to add (or remove). For example, if you want to increase its size by 15cm, you select major boost. To add this to the current potion you are making, you click on the add button directly under "Major boost". Now you can either add other (or the same!) enchantments (up to a hundred, or until you cant afford the cost), opr finish the product. For that, you click on craft (key "1"). If you could afford the cost it should appear in your Inventory, where you can select it, and drink it yourself (key shift+1). For a deeper asshole its exactly the same, except you select ass on the left box (2nd row, 2nd from the right) and choose capacity in the right box (first row, 5th item).
