Going to renovate a squeaky staircase, how do I do it?

[Skånepöjk]

Hello!
I have a staircase that creaks when you walk on it. I'm thinking of fixing this before a company replaces the treads.
But how do I get rid of the creak?
I understand I need to screw... but where? Straight down into the treads?
Screw from the other side of the staircase (from underneath) ‍♂️
Never done anything like this before, but it would be wonderful to walk up the stairs without waking the baby
Thanks for the help!

 

[Stefan1972]

if you can access the underside, isn't it easiest to approach it from there? Usually, you can see which step is moving too much and then you can secure it better in any way you prefer.