This is the easiest form of an elimination diet, although it's rarely the most efficient.
In this form of the diet, you want to cut out one or two foods that you suspect are bad. Most common allergies for babies are milk and soy, so those are often the first choice to eliminate for a breastfeeding mom.
Dairy can take weeks to completely leave your (and your babe's) system, so don't give up hope if the symptoms don't disappear immediately. But normally, if this food is a problem, you should at least start to see an improvement in symptoms within the first week. If you don't see improvements, it does not necessarily mean that your baby isn't allergic... it could simply mean that he/she is allergic to more than this one food you eliminated.
IF your babe is only allergic to the one or two foods that you have eliminated, then you are done. Quite often though, the babe is allergic to more than the one or two foods you eliminated, which makes diagnosing them much more difficult with this diet. You will have to continue taking out food after food after food to try and figure out which ones are bad.
This diet did not work for us, although it was a good starting point. I saw results within the first few days after cutting dairy. And then the symptoms returned and got worse when I subsequently increased my consumption of soy and eggs.