I have one plant 18 inches and the other one is 4".
this temperature here is between 55-68° in NY , the butts just starting to show on the fig trees.

how do i introduce it to the outside ?

I put mine out when temps are above 50 degrees or so and the wind isn't blowin too hard (like it is doing at the moment), then back in at night when/if temps drop.   I have an east facing window in the garage, and a south facing one in my dining room, both of which also get used this time of year.

i don't like moving my plants too often. i keep my new cuttings inside garage until the low is at 50. for the older tree, i should have pulled them out when low reached 40, but i waited till 50 which i think caused some issues with breba crop. the trees are now recovering from my mistake, but last all the breba and lot of leaves. i'll have to remember to take the older trees out earlier next year. 

It somewhat depends on what happens during the day. IMO if the days are warm and sunny, the plants will adjust to the night-day temps swings. I put them outside in the morning so they start with a nice sunny day, I don't generally put them out till the nights are 55F fairly consistently. If they have been out a week or two and the temps get colder for a couple nights, they seem to be OK.