To be honest, I don't really think that taking your rabbit for a 'walk' is really good for exercise. It is hard to keep up with them when they run and some don't like being restricted by the leash and won't move. Don't get me wrong, I have harnesses for all my rabbits, but I don't take them for walks either. I do agility with them, so need the harness to keep them same and have some control over them. However, if you don't have a safe yard or can get a couple x-pen, then a harness and leash can be used for some safe time outside.
First you need a rabbit save harness. The best kind are H style ones. These have a strap around the neck and one around the chest with a piece that connects them along the back and a ring either in the middle or at the back end. There are a few brands out there that sell them, but most are sold as cat harnesses. Avoid Figure 8 harnesses as they are all one piece which can tighten if the rabbit pulls and that can cause strangulation. Step in harnesses don't fit rabbits really well and are easy to slip out of, so not a good idea. Vest ones can be used, but tend to be bulky which some rabbits don't like. The velcro can get clogged with hair, so might not be good after a while.
The leash should be light weight and 4-6 ft long. I would not use a retractable leash as it causes tension all the time and can be dangerous of the rabbit runs to the end of if you drop it. A light cat or small dog one should work.
You can start by putting the harness in the cage or area and see how your rabbit reacts. Don't leave it there unless you are around as some rabbits will chew them. If they goes well, you can try opening up the straps and just laying it on his back. If he does well with that for a while (you may need to put it back on a few times), then adjust it so that it is loose but not too loose and see if you can put it one him and do it up. When you do this, have him in a safe area and be right there in case he freaks out. If he does well, you can then adjust it more to his size so it is not too loose but not too tight. Since he is young, you will need to adjust it on a regular basis as he grows.
For the leash, wait until he is used to the harness. Then attach the leash and let him drag it around. Make sure you are there so it doesn't get caught on something. You can then pick up the leash and follow him around with it. A longer one can be good if he decides to run so he doesn't get jerked back by it.
Once he is used to both the harness and leash with you following, you can try to direct him. Pull on the leash a little bit and see if he will turn to the pressure. You can use your hands as well (never use your feet to direct) so he gets the idea of what the pulling means. Keep in mind that not all rabbits will respond to this. It is good to get him to stop at the end of the leash so he doesn't hurt himself.
Stick to safe areas. You never know when a rabbit might bolt or get scared. If you do go away from home, make sure the area is safe. Grass should not be sprayed with any chemicals as they would be dangerous if eaten. If you don't know, don't go.
