Draw a typical cumulus cloud with a series of puffy lines or arcs. Remember that all clouds have a fairly flat bottom or base. Start with a straight line for the bottom and build it up with rounded arcs. Because cumulus clouds often resemble other things, feel free to shape your cloud as desired. For a thunderstorm cloud, build up the fluffy part tall and point one end like an anvil.
Draw a stratus cloud like a much longer and flatter cumulus cloud. Stratus clouds often spread across the sky. These will have the flat bottom and a flat top, with just a little curvature, so very little billowing. Draw them with a ragged line along the edges.
Draw cirrus clouds as a series of upward curling wisps. These are very thin and wispy clouds, so make the cloud shallow in thickness. Cirrus clouds are often compared to horse tails, the way they look when flying behind a running horse. Draw the lines short and curled and do a series of them together.