I don't understand

You randomly select the first person and then choose every nth person after that.

 

For example, if you want to randomly choose a group of sophomores in a high school: get a list of the names of all the sophomores, randomly choose a number from 1 to 10 (choose that person) and then choose every 10^th name after the first person. (Modify how many are chosen by how many you want in your random group.