There are several methods. If you've got an nVidea graphics card you may be able to use Shadowplay; it will always capture footage (without a huge FPS drop) and by hitting a button you'll save the last 5 minutes of gameplay; ideal for complaints.
You can also use recording software like Fraps or Bandicap. The downside is that these are not free and the trial versions are limited. You also have to activate them beforehand and it'll cause a performance drop depending on the specs of your system.
There are many different types of recording software used to record your gameplay, some of the most popular are Fraps & Bandicam.
I would also recommend you hop onto YouTube and watch some videos for more help,
Download Fraps here
Download Bandicam here