You don’t need to be able to pick him up to put a harness on him. I never picked up Bandit to harness him. Having him being ok with his front end being picked up though is very useful. While you don’t need to pick him up, I do recommend having him in a smaller area that’s sectioned off from places he could get into trouble with a half attached harness. To start with you can drape the harness on top of him over his shoulders to get him used to the feeling. I would attach the stomach loop first, before the neck loop, because if you attach the neck loop first and he panics and runs off, if it gets caught he could cause more serious damage. When I was training, I had the harness a bit loose so that when I was connecting it around his middle, it wasn’t as constrictive which is a feeling rabbits often hate. You don’t want it to be squeezing them anyway, but it does need to be secure around them. I started with very short harness sessions. Drape the harness, feed a treat, remove the harness, feed a treat. Drape the harness feed a treat, thread a loop under his belly, feed a treat, connect the loop, feed a treat. Etc etc. When you first get the harness completely on, I recommend a very short time wearing it, no more than a minute. Distract with play, treats or head rubs and then remove the harness. It can be a challenging process for rabbits that get very scared at first, but they can certainly get used to wearing one. Then if you keep your rabbit in a room with access to the backyard, all you need to do is attach the harness and leash and open the back door and out you go.
You do need to be very careful when it comes to leash length and slack on the leash. Always keep the leash slack, but you also need to train that a little bit of pressure on the leash is a good thing, not a bad thing. Sudden pressure on the end of the leash if you’re not paying attention when your bunny hops away can cause them to panic and take off running in fear, which in turn causes more pressure and more fear. If you have a secure backyard, it can sometimes be safer to let go of the leash so that the pressure is released and they can calm down.