How is a slow strum notated? By slow strum, I mean chords played on a plucked instrument in such a way that each note is distinct from the others, but they are not slow enough or far enough apart to be considered individual notes. An example of this technique is at about 1:53 in this video (listen for the harp).
On the staff, all the notes are written in a vertical line, with one stem. Just before them you need a wavy line, also vertical. It's called an arpeggiated or broken chord. Usually from low to high, but direction can be signified with an arrowhead.