Katie, the pop-up fits into the drain of the sink and is operated by a rod that connects to the drain. A pin on the end of the rod fits through a hole in the pop-up. The pin lowers the pop-up to hold the water or raises it to release the water from the sink.
Materials:
pipe wrench
adjustable wrench
slip-joint pliers


Teflon tape
  1. Remove the P-trap from beneath the sink, and remove the rod from the back of the drain.
  2. Twist off the drain flange from inside the sink, and take out the entire pop-up assembly.
  3. Clean out the sink, and place plumber's putty around the bottom of the new drain flange.
Reverse the removal procedure to install the new pop-up assembly. Hope this helps, good luck.