Use your discretion as to where to place them; most often they go at the beginning of lines. Scansion is used in music to indicate how many beats per minute (bpm) a song is played at, how long each beat lasts, and so on. It's also useful for poems since we want people to know how many syllables there are in every line (and what tone pattern they use). You can learn more about this technique in our post on scansion.