Unlike other platforms, Snapchat informs there users if someone takes a screenshot of their snaps. There is no way to mute this notification or you take a direct screenshot without knowing them. This is the reason users keep asking how to screenshot on snapchat without them knowing. As I said there is not a direct way to do this but you can try any other method. For that, you need a spare phone. Open the camera of the phone and on the other hand, go to your Snapchat and open the snaps of the person. Then you have to capture the snaps in that spare phone. You can take pictures of your Snapchat or also record the video. But you have to compromise the quality of the pictures.