Pretty much the same as KS1.
Listen to them read, with younger ones or older ones who are still struggling, I use a pen above or below the words to help them along.
If part way through already, I ask them to tell me what's going on.
Check they understand the meaning of trickier words, and that they understand the storyline etc.
Encourage expression and following of punctuation.
Encourage phonics for harder words.
Praise them (good expression, good use of sounding out, good perseverance even though it was hard etc).
Discourage them from stopping part way through a sentence even if it is the end of a page!
If you think the book was way too hard for them, mention it quietly to the teacher afterwards.
(And take mental notes which books your own DC may enjoy when they are older).