Leash training a kitten is an investment in the future.  It is worth every minute you put into it.  But, training a cat is not like training a dog.  Cats have to love and trust you to consent to training.  The first thing you have to do is gain that love and trust.  
    After that, there are a few preliminary steps.  First, start early.  Put a collar on the kitten the first day.  Get something light.  You don’t want this to weigh the kitten down and be a burden for him or hurt him.  Never, never, leave a collar on a kitten on a cat at night.  You have no idea how many life-threatening situations a cat can involve itself in when clothed in a collar.
    Halters are actually better for cats than collars.  Any self-respecting cat can twist right out of a collar no matter how tightly you attach it.  The way a fat kitten head can go through a tiny hole is nothing short of miraculous.  Besides, with a halter, you are restraining the shoulders and back, not the neck.  Any animal, including a human one, will panic if it feels its neck and air supply being tampered with.  The last thing you want is for the kitten to be afraid of the collar.  
      After the kitten gets used to the halter, attach a plain leash, a light one for cats, not one of the heavy ones for dogs.  Forget the retractable leashes.  Keep it simple.  In the beginning, the idea is that you want the kitten to get used to the feel of the collar and the leash without having it become a threat.  When you clamp the leash on, if the kitten takes off running, you want to be able to just let go.  The beginning is not the time to hold onto the leash and snap the cats neck.
    Even after the kitten gets used to the feel of the leash, NEVER drag or pull on the leash with any force.  To do so is to make sure your kitten will NEVER walk on a leash.  Walking a cat is more like him walking you than the other way around.  The cat consents to stay with you, but you will probably have to follow the cat around.  This is the best you can hope for with a cat.  Just be thankful you have that.
    Always compromise.  When the cat gets to the end of the leash and you don’t want him to go further, don’t ever yank.  Give some slack and use a “no” command.  We use an “ank,ank” sound. Our cats have learned that this means: Don’t go any further.  Change directions.”  They will often try a number of different directions, but eventually they will choose the right one.  If the cat seems to be getting frustrated or panicked, pick him up rather than allow him to fight the leash.
    As with everything else in life, try your best to be patient and loving.