The typical step up quantities are something like 800, 1200, 2000. Those are not set in stone.
Yes, chill and decant each step.
Personally, I use 1.040 wort saved from previous brews, boiled and canned in 1 qt. canning jars.
I'll pull out 1 qt., and use it for a first step.
Then chill and decant.
Then step up with 2 qts.
If I have started with a fresh yeast pack, the 2 steps is enough to decant and save the slurry 3 ways in pre-boiled water (which will be sterile and void of oxygen).
The yeast/sterile water slurry will last a long time at 34F.
With those three yeast jars, I'll use one to step up and pitch in my next brew.
I'll save the second one for later.
I'll double step up the third and start all over again.
Keg #1: Brown Bomber Bourbon/Oak Vanilla Porter
Keg #2: The Bollocks ESB
Keg #3 Lawnmower APA
Bottled: Mjolnir Mead, Lazy Daze Hefe.