Find a game's forum topic and read what the developer has posted about what they want tested, download the game, play it and leave feedback. Try to reflect on how the game makes you feel, if you had significant problems or parts you really enjoyed. If the developer has requested videos record your gameplay footage and upload to a video-sharing site unlisted. Do the same if you are comfortable sharing a video of yourself playing the game and the developer has requested it. Please try to give constructive feedback.
If you are also looking for people to play-test your game leave a link at the bottom of your feedback.