The Nexus 6P can record slow motion video at up to 240 FPS (frames per second), while the Nexus 5X only goes up to 120 FPS. Both, however, are limited to shooting 720p (720 x 1280 pixels) video when doing this.
It's simple to record slow motion video on any of the two new Nexus handsets. The phones come with Google's latest stock Camera app, which may not offer a myriad of features, but it's straightforward and easy to use.
Note that, when shooting in slow motion, the output video will be darker than what you get when you shoot at 30 FPS. Thus, for best results, you should make sure that the scene is well lit. When you're done recording, play the video, and tap anywhere on the screen to edit it and select what part of it you want to be in slow motion. Of course, if you want, the entire video can be in slow motion - there's a slider that lets you adjust the start and end time of slow motion.