Honey isn't 100% fermentable by beer yeast so it leaves flavor behind. Add about a 1-2lbs, depending on how prevalent you want the honey taste. Add it like you would any LME.
This a very misleading statement. Honey IS pretty much 100% fermentable, and not all kinds leave a flavor behind. It all depends on what type of nectar the honey is made from, and with the exceptions of a few types, the nectar source is rarely controlled.
Chances are, if you use 1-2 lbs. honey in a recipe, you are going to get a BIT of the flavor in your beer, but it is guaranteed that your are going to boost ABV and DRY out the beer. This ABV boost can throw a good beer recipe out of balance where you taste too much of the alcohol and not enough of the body and bitterness, and a dry beer is generally only a desired taste in certain styles, like some wheats, hefes, porters and stouts. If you want to go that route, add your honey at flameout after your boil is complete. I've had the best results with orange blossum or wildflower honey.
Honey MALT gives a much more controllable and definable honey taste to beer without the unwanted side effects. 0.5 lbs of honey malt per 10 lb grain bill gives a subtle honey flavor, anything over 1.0 lbs per 10 lbs grain bill is too much, so you can use to taste between 0.5 and 1.0 lbs per 10 lb grain bill (or 5-10% of total grain bill if you want to think of it that way).