Collect a plankton sample. If you live near an ocean, a plankton net can be used to collect live samples. If you don't live near an ocean, preserved samples can be ordered from a science supply store.
Collect a few drops of your sample in a petri dish using your eyedropper. Make sure the sample is in the middle of the dish so it doesn't spill over.
Place the petri dish under the microscope and observe the plankton. Since plankton can move up and down in the sample, you may have to refocus your microscope to see plankton at different levels. As you observe your sample, take notes on its characteristics and draw a detailed picture of what you see.
Compare your drawing and description to your Plankton Identification Chart. In most cases, you will be able to tell right away what kind of plankton you're observing, as well as the specific species. Sometimes you will have a bad sample, however, and identification will be more tricky. If this is the case, move on to the next step.
Determine if your sample is made of plant cells or animal cells. Since phytoplankton is a plant, its cells will be square with a stiff cell wall. Zooplankton is an animal, so it will have round, soft and flexible cells. If the cells of your sample resemble a plant, then you've identified phytoplankton.