Preparation of the 灰型 can happen the night before, but the ☵ is only written just before the coal is laid, to allow more air in. Also, フジ灰 you add then, too.
You should also have a back-up 釜 running, so in case something happens with the fire, you still got boiling water. In that case, you use straw ash (わら灰).
炭
Fill in the coals as if they were growing from the ground; so you can check for branch holes, or just see which side is thicker (= bottom).
えだ炭: try picking them up.
Rattle it slightly to see if the coals hold.
はいき: Don't tap the spoon on it, since it is quite fragile.
茶碗
Practise cleaning once if it's a new bowl
茶入
See that 茶杓 can rest on the lid
Make sure knot is loose enough.
棗
Check front
In III 20, I found it nice if there's the slightest dust of tea powder on the 棗 which can then be dusted off
Generally
Prepare for the 拝見, don't forget to ask about the tea.