Picnic cups make great containers for starting young plants. They are tapered, so that the plant slides out easily when the cup is inverted. They are transparent, which lets you see how the roots are developing. They come in different sizes and they are also relatively cheap.
Many people see these plastic cups as disposable, so after the party they all go into the trash. That sort of non-recycling attitude brings bad karma on the planet. Help absolve people from their sins by rescuing these useful cast-offs and putting them to good work in the garden. Then again, you can always buy them new if you need them badly enough and no parties are in sight. As I said, they are relatively cheap.
The cups can be used several times before the plastic finally becomes brittle and breaks.
Step 1: CUTTING THE DRAINAGE HOLES
You need a sharp knife to cut the drainage holes. I hold a stack of cups in my hand when I do the cutting. When the cups are stacked they are more rigid, which helps. A single cup flexes more and is harder to hold.
For safety's sake, always cut away from yourself. With practice a little flick of the wrist does the job quickly.
Nick the bottom edge of the cup three times. Be careful not to make too big a hole, or potting soil will come out. If the holes are too small, there won't be enough drainage and roots will drown in water.