The best way to get started is just selecting a couple seconds for the previous week. You can always go back and delete the ones you don't like. This gets you comfortable with 1SE and having fun right away!
Orange triangles are your friend!
You can tell if you have a content on a calendar day because it appears orange. A video will show a camera icon also. Tap on any of these days to see the videos available to grab a second from.
Use the Arrows, Luke!
You can use the green arrows in the middle, to go frame by frame, in order to find that perfect moment. Don't forget, you can preview your second by tapping the green square on the scrubber.
TIP: You can also use "Flux" to grab a video or photo from ANY day on your phone, then use it on a calendar day in 1SE. Flux is the orange circle with a plus that's viewable inside a calendar day.