I don't know if you have seen this thread - Walking the cat?
It might be of some help to you!
Both my two kitties walk on harnesses and leads! It does take a while for them to get used to it, especially if they are older. (But it is certainly not impossible!
To start with, put the harness on kitty for about five minutes and let them wander with it on. Build up the time that you leave it on, over a number of days. (Kitty will generally tell you when they've had enough!) Tibby got jelly legs at first, everytime his harness was put on! He soon got used to it though!
Once you have got the harness sorted, attach the lead and let kitty lead you around the house. This will get them used to being followed. Again, build this up too and don't try to control the cat! (remember, they are your boss!)
Once this is sorted, you can think about going outside!
Choose a door that you will always use when you take kitty out for walks - this way, the association with going outside is only attached to this door. (it is preferable to be one you don't use often)
When you go to take kitty outside, carry them out - this way, they learn that they can only go out when they are carried. Once outside, let kitty lead you - as you did in the house - and let them sniff and wander as they please! (It can be a bit boring when they find a really good 'smell' as you can be there for a long time!
When it is time to come back in, let kitty go through the door themselves - ie walk through - this way, if they do become scared or the lead slips your hand, they will run to the door for safety! (I have had experience of this!)
I would recommend trying to keep the outside time to a constant time - ie 30 mins or so. This way, kitty knows when their time is up!
I hope this has helped you a bit! Keep us informed as to how you go, i'd love to hear about it!