I usually add it either during the chill down or once the batch has been fermenting for about a week, or two. Simply warm it up in a water bath, IF you must, but keep it under 100-110F. Then just pour it in. You can usually get all of the honey in. You might leave a little behind this way but not much. IF you really want to you could add a few ounces of warmed water to get it to flow easier.
How much really depends on what you're making. Do keep in mind that you can get more honey character by using honey malt in a brew.