All the cats I have harness trained have been in their 1st year.
I started by putting on the harness for short spells though out the day, (never leave them unsupervised at this stage)
Please use a cat harness not a dog harness, there is a difference and a cat can get out of a dog harness,
see cats are smarter than dogs
slowly increase the time of wearing it, its a good idea to make this time play time, it will distract from the harness aswell as make it great fun.
After a few days I attached the lead, which was quite amusing as he thought he was being followed about and all the other cats were pouncing on it.
I then thread the lead onto the washing line this lets the lead slide along easily when he goes walking round the garden.
I took out a blanket for us both to sit on and we had a great 1st day.
Another good item to attach the lead to is a tennis swing ball pole.(dont tie lead, thread lead.)
Once they get use to the washing line then you can slowly venture out of garden.
Its great to see peoples faces when you mention you have the cat out on the washing line
Dont forget to do a quick assesment for dangers ie, cant jump over the fence and hang themselves, tangle themselves up on chairs ect.
My cat looks forward to going on the washing line even though they do have a piece of the garden secured off for them.
GOOD LUCK AND LET US KNOW HOW YOU GET ON