Besides the creating the flyer, how will you collect the items? If you are collecting donated items, you have to make it as easy as possible for donors. If they bring something to work with them, you'll have to provide something they can quickly unload as soon as they get to their respective offices.
You can either have a large bin, box or basket your building's office workers can put things in, either at your own office entrance or your building's lobby (as long as no one will walk off with it). Or, if its not alot of offices, you can provide a container for each office which can be left at their front desk, and you can stop by each day and empty it for the next day's donations.
What a good thing you are doing to help people. I admire you.