Don't be nervous, they know what they're doing! It's just hard for us because we can't grasp the concept of not eating and going to sleep for 3 months (wish I could but, in winter!).
You could still leave all your heat and light settings the same, but if your bluey wants to sleep then he will anyway. Or, you might reduce them all but he just doesn't want to sleep.
Or, if like my Jubby this year, he was active active active .... and then the next day he was gone. So after a week of not seeing him I just turned all his lights off. Didn't even get a chance to reduce them.
I would stop taking him out, and I'd brumate him indoors.
As a rough idea, my daytime winter house temp fluctuates between 64-71, but can get as low as 54. Nightime, my house temp is somewhere between 50-60.
To give you an idea of what I generally do:
If after offering food for about a week but he's not eating, that's my indicator that he wants to sleep. I'll start reducing his lights by half hour in the morning and half in the evening, keeping reducing over the next couple of weeks until they're down to about 10am to 3pm, and eventually when there's no sign of them then I turn them off alltogether.. For mine that are in a high traffic area, I either put a blanket over the tank or move them to a quieter room.
Relax! It's not a definate science! You may well get all the lights down and then, surprise!, you wake up one morning and there's a skink sitting there in the dark! No dramas, just pop a light on for as long as he's up and then off they go again when he goes back to sleep. No need to offer food, they will be TOTALLY fine without it for many months.
It's all play by ear stuff. Take hints from your skink. Turn him off, if he gets back up again, turn him on again. They're all different!
I understand your fear! We've all had it first time.
But seriously, you won't harm your skink if you give it a go and skinka decides not to.
Two things, always let them have access to fresh water, and don't brumate a skink on super full tummy. Like, reduce your heat gradually, but don't go the complete turn off if he ate a plateful yesterday.
You really do have to take cues from your skink, such as reduced appetite and lots of sleeping. Generally a sign that they want to go to sleep.
Oh and finally, some sleep for 3 months and some sleep for one. Some go out like a light and some do the sleep for 5 days, wake up for two.
Brumation is a natural thing for them. I think they need the down time. As much as I miss mine whilst they're kipping, it's what they do. It's their way of coping with cooler temperatures.
P.S. If you're really worried about missing them, do what I do and just get a new baby each year
I'm gonna get killed for that last comment.