Typically, the stopper is attached (by a hook in the end) to the actuating arm that moves the stopper up and down. Reach under the sink and unscrew the threaded cap where the arm goes into the drain (on the rear of the vertical drain pipe just under the sink). After you pull that arm out, you should be able to get the stopper out. Clean the stopper and reassemble with the arm going through the little hook in the end, rethread cap hand tight, and you should be good to go.
Easily distracted by shiny objects.