Soldering electronics is a delicate art. If you're just getting started learning how to solder, this handy reference chart can help you spot some common mistakes and make sure your work comes out right.
The chart above from Adafruit is no substitute for a proper tutorial (though they have some of those as well), but it does pack a lot of information into a small space. When you solder a joint, you want to heat the pin and the pad for 2-3 seconds before you introduce your soldering material.
If you don't heat a part enough, heat it too much, or don't add the right amount of solder, you can get some wonky looking joints. The right side of this chart shows how your joints might look if you mess up and what the problem was. The left side, meanwhile, reminds you which tools you should be using. For example, you should never use a soldering gun for tiny, detailed electronics work.