When you open the drawer and feel underneath, you'll feel a cutout of the shelf the drawer sits on.
There is a little spring lever along side the slide, and you need to depress is so the back of the drawer will slide over it.
If your fingers are thin enough, you can use them, otherwise a screwdriver or something like that would work.
It's easier if you do with with the cabinet on it's back - or upside down, especially for the bottom drawer.