When my kids were younger and each was in Preschool, which they attended for 1/2 days part-time... they both napped once we got home. It was not forced naps and they needed it and knew it.
Then, when my older child for example, outgrew naps, then it was quiet time, while her younger brother napped in the afternoon.
How?
I just said "afternoons are quiet time... and brother is napping. It is quiet time for Mommy too."
And that's it.
My daughter understood.
She was 4 at the time. My son was just born etc.
And it was fine.
Afternoons were quiet time.
No "method" or formula to it.
But on days when even my eldest was tired, she would nap, too, if she needed it. And she would do so after school.
I did not use TV for quiet time. I did not confine them to their rooms for quiet time.
Quiet time for us, even me, was just spent anywhere in the house, as long as it was quiet, things. And whispering.
My kids could do what they wanted, as long as it was quiet, things.
And as far as "signaling" that quiet time is over.... it does not have to be complicated. You just tell your kids "quiet time is over."
That's it.
It doesn't require methods or gadgets or anything complicated.
It just takes, you as the Mommy, telling your child. And then, having a daily routine, about it.
Both my kids, KNEW the afternoon routine.
Because it was a routine.