First, unlock the stage by pushing the lever towards the back.
You may or may not hear a click. If it does click, the stage was locked at some point in its travel.
Next, place a slide on the stage.
Rotate the 100x or the highest power objective in place.
The reason for a stage lock, is to protect your specimens from crashing into the objective causing damage to your specimen and your microscope optics.
Next, carefully raise the stage up to the point where the objective is at the working distance it needs to make an image then go a little beyond to make room for imaging at different focus points.
Be mindful that the objective does not touch the slide or coverslip that is on top of your specimen.
Next, move the lever towards the front until it is snug.
The stage is now locked.
Rack the stage down then up.
Notice that the stage will not travel farther that the position it was locked.
To unlock the stage again, push the lever towards the back.
You will hear a click signaling the stage is now unlocked and can travel its whole distance.
If working with the same specimens which have the same height, the stage will not need to be unlocked and reset unless or perhaps you have specimens with a different mounted height or depth.