I love volunteers! The way I set it up was to find out if they had a specific time/day they wanted to come in (some prefer a set schedule). Then I made sure I had specific assignments for them. You can even make a tray for them with things to do (copies to make, things to laminate, bulletin board to put up/take down, papers to file, etc...). You can even make a form that you just check-off the job (copy pages ____, cut out/laminate the words ______, etc...) That's the easiest way. There's always a break in class time that you can discuss any questions they have. If you have several volunteers, you can either have one tray for them to pick from, or if they have specific things they'd rather do you can have a tray for each volunteer. Also, if you have science experiments or something that needs more adult helpers you can ask for them at that specific time.