Have you ever run across needing to solder multiple small PCB boards at once? If so, you know how much of a hassle it is to do them one by one. There are some ways that make it easier to do. For instance, using a solder pot. Using a solder pot to solder smaller Thru-hole boards is easier than doing them by hand soldering. All you need is a wide solder pot, a 63/37 or SAC305 (Lead Free) bar solder, and Rosin flux.
Here is an example of a solder pot that can be used for this task:
Take a good look at the bottom of the board and check the connection pins. Apply solder to the joints you don't feel comfortable with. Other than that, you're done! Its that simple and allows you to do this for multiple boards if necessary.