For carrots, I use our rice cooker as a steamer by putting a half inch of water in the bottom and then a strainer in on top. I cut the carrots into round slices, put them in, and then turn the rice cooker on for about 20 minutes.
When they're soft, I pour them into our food processor, add the water from the bottom of the rice cooker, and then add in about 2 ounces of breastmilk as well. I then blend them. Carrots don't tend to get really pureed; i.e. they'll have a kind of bumpy texture. I just make sure that there aren't any pieces bigger than a blueberry.
Then, I spoon the contents into ice cube trays, cover with saran wrap, and place in the freezer. 6-8 hours later, I pull them out of the ice cube trays and place in a covered freezer storage container. When I want to feed my daughter carrots, I pull an ice cube carrot out of the freezer, heat up, and serve.
For other purees, I do basically the same thing. I heat/cook the vegetable or fruit, blend it with breastmilk, and freeze.