If you are the mother of a young baby, under 4 months old, I recommend not yet experimenting with diets. If you can hold off, maybe even wait until their are six months old. At such a young age, the breastfeeding relationship is still iffy, and some women might struggle even without doing some dieting that may affect their supply.
If you cannot bear to wait any longer, and want to start being healthier NOW, then I highly recommend you start with "maintenance mode" of the ketogenic diet. Rather than limiting yourself to 25 grams of carbohydrates in a day, increase that to 100 grams in a day to lessen how drastic of a change it will be. From there, slowly decrease your carbs over time.
In any case, here are some tips.
Tips for Breastfeeding Women on the Ketogenic Diet
1) Keep an eye on your breast milk supply. This is the most important. Because everyone is different, things affect people in different way. To be on the safe side, make sure to keep watch over your output, especially the first couple weeks getting into ketosis.
In my experience, because of my crazy carb-laden (junk food) diet prior, I had a near-constant over supply. This went away when I began having a regular, steady diet of low carb foods. I still produced enough for the little guy, but no longer would have crazy letdowns soaking through my shirts at a moments' notice.
If you DO start to notice a dwindling supply, or if you are nervous about it at all, I recommend maintenance mode: 100 grams/day of carbs rather than 25(see above).
2) Drink TONS of water. Really, this is just as important as number one in many ways. It is important to keep hydrated as a breastfeeding mother, and even more important with the ketogenic diet. This is because carbohydrates, when consumed, grab hold of water molecules within the body. By getting rid of carbs, you are losing this water weight and will have to drink possibly twice the amount of water you drank before.
3) Eat enough (of the right type of) calories. I know, you're thinking to the numerous times you've heard "calories in, calories out" and thought that by eating fewer calories, you will lose weight more quickly. However, the body doesn't always work like that. After a few days of under-consumption, especially as a breastfeeding woman, your body may react by trying to prevent weight loss. Think of all the people you know who tried to consume 1200 calories per day and still couldn't manage to lose weight.
How to know how many calories you need? Here is a handy online tool:
This also tells you how much of what types of calories you should consume in order to hit your macros! Really helpful, especially in the first few weeks.
4) Keep track of ALL your foods and drinks. I highly recommend MyFitnessPal (and their free Android app) to keep track of your intakes. They have an EXTENSIVE database of foods and drinks, and if you can't find your food there, you can grab the nutrition label and add it yourself. They even have a huge number of restaurant nutrition information! Really very helpful. If possibly, invest in a food scale. This will make it much easier to figure out how much of a food you are truly eating.
5/24/15 Update: Newer post here!