Not using OLED sockets? Skip ahead to Batteries.
Insert the socket from the top of the PCB. You can use a piece of electrical or scotch tape to hold it in place.
Flip the PCB and solder one single pin of the socket.
Then, flip the board around again and check whether the socket is aligned straight. If it’s not, heat up the solder joint and move the socket around until you’re happy with how it looks. Lastly, solder the other three pins.