We'll start by adding a "blob" of solder to the battery pad on the back. Heat up the pad (it looks like a silver disc) for 2-3 seconds, feed in some solder, and spread it around with the tip of the iron. It should look like a shiny, flat puddle that covers the entire surface of the silver pad.
This will act as a "cushion" to help keep the battery in place later.
Next, arrange the battery holder on the back of the pin. Place the little "arms" of the battery holder through the tiny holes in the PCB. It should NOT be flush to the PCB, but sit on top of it like a table, leaving space for the battery to be inserted later.
Gently flip it over onto its back, and place it on a flat surface. Solder the battery holder in place by heating up the "knee" joints on the front of the PCB and applying solder where the points of the battery holder peek through the knee joints.