Our kitten is seven months old and we have big gardens. Its a bit tricky to keep him in as we have an old cat and two dogs, so, this is what I did:
Carried him outside a few times (over about a week) and just walked round the back garden with him then brought him in.
Sat outside a few times (again over a few days) with him on my knee watching the dogs play, he was looking everywhere, at the birds, at the wasps, watching the trees swaying etc.
Then put him on the back lawn whilst I was gardening and made sure he stayed in the garden.
Everytime we brought him back in we gave him some treats.
We are home all day so he plays in the back garden with the back door open and up to now has always comes back when shouted, and is always given a big cuddle and some treats so he knows its a good thing to come back.
The cats have to be indoors when we go out and definately stay in overnight ( big cat is 15 and has never been out overnight).
I also mentioned to the neighbours that he was ours and to shoo him back to our garden.
It is a bit nerve wracking and I'm dreading the time he doesn't come back straight away.