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The only people I've ever heard of that trained a cat to walk on a leash either started the training when they were very young kittens; or they were at home all the time to work extensively with the adult cat.
I don't recommend using a 'dog' collar and leash on a cat; due to their physiology and muscle structure, I recommend a full body harness.
The first thing you must do is to desensitize your kitty to the harness and leash by first playing with kitty using the harness and leash. Leave the harness out for several days/weeks so that Kitty is not afraid of it.
The next step is to put the harness on kitty, but he/she won't like that so you must slowly increase the time you leave the harness on to desensitize Kitty to this 'unnatural' feeling. Start with just a few seconds, then increase the time a few seconds a day until Kitty doesn't think anything of the harness. Always follow 'harness training' with a food treat, so Kitty will associate a good thing with wearing the harness.
Once Kitty will wear the harness with out too many complaints, you can start the same process with the leash as you used the harness. Slowly increase the time the leash is attached to the harness until Kitty is fairly nonchalant with the leash. Always follow a leash/harness training session with a food treat to reinforce a positive reaction with the training.
Once Kitty is comfortable with both the harness and the leash, then you can take Kitty outside for the first walk.
I don't recommend trying to use the harness and leash until Kitty is completely trained, otherwise Kitty could panic and try to escape. The leash could get caught on something and cause even more panic, which is when most kitties can get seriously injured.