For instance,how to make a grapefruit IPA.Do you use peel or zest or flavoring?When to add.Boil,flameout,fermenter,keg?Blood orange is another one i'd like to do.As far as amounts,i can probably figure that out.I'm a long time all grain homebrewer.
Many commercial examples will use the whole fruit puree peels and all. Hangar 24 orange wheat for example.
Sam Calagione has in his book Extreme Brewing a great chart for what temps are best for most fruits. Most are added at about 155-169°F after flame out, then held for 20 minutes to pasteurize and get flavor extraction.
I would only add citrus puree at the above temps, not to cool wort or in secondary. To prevent wild yeast from taking hold.
Only use dried peels for secondary additions. You can make your own in the oven to insure wild yeasts are killed.
First of all, pick hops that will accentuate the flavor you are after. There are several hop varieties that can lend citrus/grapefruit flavors: Citra, Cascade, etc.
Next, when I have brewed with citrus, I added zest at knockout. Let it steep for a few minutes with the heat off and then leave it in as you run the beer out to a chiller.