Take it slow.
That is the best advice you will ever get.
Introduce the harness that you plan on using well before attempting to put in on your cat.
Make the harness a good thing (bring it out during feeding times, playtime, etc). Heck, just leave it out so it becomes a part of everyday life.
When your kitty is used to its presence, put it on. This may take some time (I honestly suggest a harness that has separate clasps for the neck and chest, it makes it sooo much easier).
Make wearing the harness a good thing; give your kitty treats, play with him, love on him. Make everything about wearing it positive.
Next up is introducing the leash. Make it a part of the environment just like you did with the harness. After he's used to it being around, put his harness on and attach the leash (follow him around the house holding the leash). Again, make everything about this extra weight positive.
Once he's used to having the harness on with the leash attached, you can take him outside.
Just remember the cat walks you, and it's generally more of a casual stroll.
I recommend a Kitty Holster for the harness (you just have to make the sound of velcro a good thing) because it's harder to escape than most, and a retractable leash (one that's built for small dogs)
Also, your cat will probably act like he's been crippled the first few times that he has the harness on... It will be both scary and hilarious. Play is crucial to get him used to the idea that movement is possible while wearing the harness.