For herbs, it's best to use fresh herbs. My basic rule of thumb is to use 1/4 cup of chopped, packed herbs for every 3 cups of dairy in the recipe. Of course, it depends on the herb. Go easy on strong herbs like rosemary, and go heavier on leafier herbs like basil. For spices, the amount varies. One way to think about it is how much of the spice would you want to brew like a tea in a 1-cup (8 oz) mug of water? For example, for star anise, I might put two or three pieces in. For cinnamon, maybe a teaspoon at the most. Use this rule for every cup of dairy in the recipe.
1. Begin with the basic custard or non-dairy base. Each of these has as its highest quantity ingredient a liquid. Milk, water, juice.
2. Simmer the herb or spice in the liquid. For a custard base, you usually begin by cooking some or all of the milk anyway. This is the time to add the herbs or spices. If there is no cooking, as in a granita, simmer the base liquid anyway with the herbs or spices, then bring to room temperature before proceeding.
3. At this point, some infusions need to be puréed and or strained. With the basil ice cream I made this week, I let the infused milk cool, I put it in the blender, then I strained out the solids. If you've infused the base with a cinnamon stick, obviously, you would simply remove the stick. Use common sense. Think of people's teeth after they eat the ice cream. If any blending or straining needs to be done, add that step. For the recipe to read visually, you can always use whole or ground versions of the infusion as a garnish: a sprig of basil, a sprinkle of ground cinnamon or a cinnamon stick plunked in the scoop like a straw.
4. Continue with the recipe as written.