Step1
Hay
The key element in any rabbit's diet is hay as it provides the fiber necessary to keep the digestive system moving. Grass hays, such as timothy, are best as they are lower in calcium, fat, and protein. Alfalfa isn't recommended past the age of 6 months. Hay should always be available to your rabbit. They can receive unlimited amounts. I usually place the hay in my rabbits' litter boxes, as they often like to poop while they eat. No need to worry about them eating hay they've urinated on; they won't do so under normal circumstances (that is, when they're provided unlimited amounts of it). Just give them a fresh bunch every day (or twice a day).
Step2
Greens
The other thing rabbits should be given daily is fresh greens. Ideally, your rabbits would be given three types of fresh greens a day. If you can't provide that, please at least give them one type. Also, it's important to wash the greens as they often contain dirt, pesticides, and other chemicals. Do NOT give your rabbits iceberg lettuce. It is made up mostly of water and will cause diarrhea. The following greens are acceptable for rabbits and they should be given about 2 cups of green per 5 lbs of body weight:
kale
dandelion greens
collard greens
red leaf lettuce
green leaf lettuce
romaine
escarole
chicory
dill
cilantro
parsley
Swiss chard
basil
oregano
mustard greens
carrot greens
Step3
Pellets
Contrary to popular belief, pellets are not the main part of a rabbit's diet, or at least they shouldn't be. They are the leading cause of obesity and malnutrition in rabbits. Adult rabbits should only receive about 1/4 cup of pellets per 5 lbs of body weight. It is important that you pay close attention to the type of pellets you buy for your rabbits. Anything that contains items in addition to the pellets (such as seeds, nuts, etc.) should NOT be given to your rabbit. Look for a hay (timothy) based pellet. Like human food, rabbit food contains nutritional information on the back of the package. In a perfect world, the pellet you give your rabbit would have less than 2% fat, less than 14% protein, and at least 20% fiber. Unfortunately, there aren't many readily-available pellets that contain all three stipulations. Try for at least two out of the three.
Step4
Treats
Treats should be given sparingly, as a rabbit's digestive system can be easily upset by excess sugar. Other than vegetables and some fruit, rabbits should NEVER be given any kind of human food. They also shouldn't be given any of the "treats" sold at pet stores as they are high in sugar and contain many ingredients a rabbit's digestive system is not supposed to handle. The best treats to give them are fruit. Mine love banana and raisins, in particular. You have to be careful with raisins, though, as they have a high concentration of sugar. Since it is naturally occurring sugar (fructose), it's OK for them. Only give them about 2-3 each, though. If it's a banana, only give a small slice, no more than 1" long. You could also give a small slice of apple, a few dried cranberries, or other small quantities of fruit.
Step5
Water
Finally, always provide plenty of fresh water.
The basis of any healthy rabbit diet: hay
Hay
Greens
Pellets
Treats
Water
Novice 
