ChileRelleno pretty much covered it. Just be sure to take out any uneaten food before it spoils. If there is no algae left in your tank, it shouldn't take them too long to notice the fresh veggies. As I type I am watching my zebras munching away on a cucumber slice. For wafers, I recommend the HBH brand veggie wafers. I prefer them over the Hikari algae wafers, and so do my fish (the first ingredient in the Hikari brand is fish meal, in the HBH it is spirulina).
Oh - as far as how often to feed them, if there truly is no algae for them I would give them a constant supply of veggies. I give mine a variety, including zucchini, cucumber, baby spinach, romaine lettuce, and the wafers. Take out the uneaten portions each day and replace with fresh. I use plant weights to hold the veggies in place, but you can also rubberband them to rocks or driftwood, hold them down with rocks, etc.
Another option is to start an "algae farm". You can put some rocks in a container near a window that gets lots of light, and once they get covered in algae just drop them in the tank. Once they are cleaned, drop some more algaefied rocks in and put the clean ones back to grow more algae.