The idea is that a task is broken down into smaller steps. Each step has its own picture or word, and these pictures/words are put in order, left to right, on one strip of velcro. There is another strip of velcro below or aside the strip with the pictures, which is empty. The whole skill or routine is essentially laid out for the person on these pictures. Starting with the first picture on the left, they complete the step on that picture. When that step is done, the person moves the picture to the empty velcro line. Then they move to the next picture, and complete that task. And so this continues, step by step, until all the pictures are on the other velcro line.
In our house, we have a picture schedule for his morning bathroom routine. These include hanging up pajamas, brushing hair, washing face, putting on robe, etc. There are several for his evening routines: a set of pictures for his pre-shower (clothes down clothes chute, get bath towel, put down bath mat, turn on water), a different set for his actual shower, and then another for after the shower (dry off, brush hair, brush teeth, floss teeth, put on robe). We are seeing great progress in him becoming more independent with all these routines, thanks to the help of these picture schedules. The intent is that they will no longer be necessary - some day. But for now, without them, he might get started on a routine but forget what comes next, get stuck partway through and just stop. Or he might not know what to do to start a routine. The pictures help him move through these long tasks and gain independence.
What's really satisfying and simultaneously a little heartbreaking is that once he gets a routine down, he really doesn't want us hovering nearby. It's as if he's saying "Hey, I can do this! Get off me!" Very age appropriate though - just like any 12 year old wanting her/his independence and not having Mom or Dad pestering!