Put it on him, and leave it on.  Give him treats w/it on.  Let him get used
to it.  When I get Oscar's (rescued runt Siamese) harness out, he starts to
purrrrrrrr like mad.  He knows what it means now.  At first, he would lay
down and not move.  Then he walked around a wee bit in a crouch.  It took a
while!!  :o)

Skritches and skratches to Harley...   ,,,^..^,,,

This is only an idea.  I have no experience in harness training kitties, as
mine are safe to go out and therefore never needed it.
I do, though, have a lot of experience with lead training puppies.  This is
what I suggest.
Put the harness on your cats while they are in the house.  They might not
like it and will probably roll around trying to get it off, but ignore it.
Leave it on until they have forgotten it's there and then attach a lead
which will drag behind them and they may not like it at first.  Ignore this
too, but provide reassuring words at all stages.
Pick the lead up.  It's a good idea if you let them get a little bit hungry
and take them to the food with their lead and harness on with you holding
the lead.  Take it from there, slowly.  Next step, looking out of the front
door with harness and lead on.  Then a little trip into the garden.  Etc.
Hope it works for you.

Tweed

Put harness on as part of regular life, like a collar (although in fact,
I didn't leave it on all the time). Let him discover indoors that the
harness does not anchor him to the floor. Then attach one end of the lead
to the harness and the other end to a stationary object indoors; let cat
adjust. I did this while desperate for a way to cut out a pattern on the
floor of an apartment with no cat isolation area except the bathroom, the
door of which was showing the effects of determined exacavation efforts.
You may consider cutting out a pattern within sight of a cat who can't
reach it cruelty to cats.

Finally, allow cat out in sheltered area on leash and harness. Never leave
cat unsupervised (you will be astonished how easily the cat will manage to
get tangled in a bush or fence you could have sworn he couldn't reach). At
this point, the cat generally makes the connection between Out and
Harness.

I have to confess to a 50% success rate. Well, 100% at wearing the
harness, but Betsy is just humouring me. I can tell, because she always
wriggles out of it when she wants to go inside!