i have also heard of sticking the turtles in the fridge, but never thought it a very 'likely' option. I don't think that you will get many answers on this forum because as far as i know, most of the people here keep their turtles active throughout the year.
Although i know nothing about hibernation, i would say to lower the temps of the tank slowly.. and maybe cutting the amount of light he/she gets per day... then one day just not have the light come on at all... you want to simulate the 'real world' as much as possible... and considering that it usually gets cold 'slowly' and it gets darker by day 'slowly' i would say gradually is the best! i'm not sure if there is such a thing as 'too cold' for the turtles, but you definitely dont want him/her to get any infections.
I would also suggest looking in on him/her if you do decide to hibernate because you never know when they might kick out of it and be HUNGRY! also, i wanted to say that i read somewhere (not really sure where or when) that turtles hibernate in the mud in or around ponds? i can't remember where i read this... and i honestly cant say if it's true or not... but it may help you!
i hope any/all of this helps 
-----
~ Evie 
Pets:
RIP 0.1.0 German Shepherd {Dusty}
0.1.0 German Shepherd/Collie mix {Shadoe}
0.0.2 RES turtles {Booger & Snot}
1.0.0 African Clawed Frog {Bingo}
0.0.2 Snail {Gary & Larry}