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 days are your friend!
You can tell if you have a video on a calendar day because it appears orange. Tap on any of these days to see the videos available to grab a second from.
Use the Arrows, Luke!
You can use the blue arrows on the bottom left and right to go frame by frame in order to find that perfect moment. Don't forget, you can preview your second by pressing the tiny play icon on the bottom left in the orange rectangle.
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 top right icon when you're inside a calendar day.