It is not particularly easy to take a rabbit for a walk on a harness and much time and patience is needed to train a rabbit to do this...
The best form of exercise for a rabbit will always be free run in as big an enclosure as possible. This allows them to exercise at their own pace, pausing when they want to which is important as rabbits tire easily.
It is not particularly easy to take a rabbit for a walk on a harness and much time and patience is needed to train a rabbit to do this. However, it is possible and rabbits can learn to enjoy this form of exercise, particularly if there is no other form of exercise open to them.
Type of harness
There are two main types of harness available - those which are like a jacket with velcro fastenings and those which are straps with buckles. The important thing is that the harness goes around the rabbit's body as well as around the neck - never use just a collar and lead on a rabbit as this may snap its neck.
Most harnesses are adjustable and are available in different sizes. You should be able to fit two fingers in between the harness and the rabbit's body at all points but it must not be too loose or the rabbit will just wriggle out of it.
Training a rabbit to a harness
Firstly, detach the lead, put the harness on the floor and let the rabbit approach it and sniff it. Then pick your rabbit up and hold it securely while you put it into the harness. It might be helpful to have some help for the first time you try this. Ensure the harness fits correctly and that your rabbit is not too distressed by it. Put your rabbit down on the floor and let it go. It will take most rabbits a while to accept the harness and they will probably try to pull it off or nibble at it. Leave the harness on your rabbit for a while, 20 minutes or so, then remove it. Repeat the process for a few days until you are confident that your rabbit has accepted the harness and "forgotten" about it. Do not attempt to attach the lead until this process is complete.
Once you are happy your rabbit has accepted the harness, you can attach the lead to it. Keep your rabbit in its own environment i.e. don't take it outside immediately. Let your rabbit hop around without restriction; follow it so that the lead is never taut. Watch for signs of stress in your rabbit - fast breathing, bulging eyes and thumping its back feet; if at any point you feel your rabbit is stressed release it from the harness.
Once your rabbit is accustomed to both harness and lead, you can take it outside for the first time. Your own garden is ideal for this but if you don't have a garden try and choose a quiet park with no dogs. Rabbits don't like open spaces and prefer to be under cover so don't be surprised if your rabbit heads straight for the nearest tree or bush. Don't attempt to halt it with the lead at first; just follow it and crouch down and stroke it frequently to reassure it. In time your rabbit's curious nature will get the better of it and it will start to look around and investigate its surroundings.
Going for walks
Don't expect your rabbit to just hop along beside you like a dog would. A rabbit's natural gait is to hop a few steps, then look around for danger. This makes it a very time consuming process to walk them! However, a confident rabbit who has absolute trust in you will most likely happily amble along with you at its own pace and enjoy the outing.