Every so often I cook some beets to get some different colors into my daughter's diet - like greens, reds, yellows, etc! Just remember that eating beets can definitely have an affect on what colors you see in your baby's diaper, so don't panic!
You can either steam or boil beets. I peel and rinse them, and cut them into slices first.
I used to do just plain beets, but I have also mixed them with oranges and turnips. I actually like the idea of turnips, because when I freeze the beets as meals, they tend to be very dark, and it's tough to distinguish the beets from other dark meals (like prunes or berries), so adding some lighter vegetables helps make them a little "pinker".
After cooking, I simply puree in a blender. You can add a little water if you need a thinner consistency, but I try to keep the beets as thick as possible so as they are the least messy - beets stain very easily and especially with toddlers (like my 20 month old that refuses to let me feed her). If you don't want to "water down" the beets you can use fruit juice, although the beets are generally pretty sweet on their own.
The beet puree freezes very nicely and heats up well in the microwave. I generally do a small side of beets with some other dinner foods (like grains or pasta).