I had way too much fun photographing these lemons, so I apologize for all of the yellow that is about to fill your monitor and cause you to retrieve your sunglasses.
Aren't lemons pretty? Oh, right, you are here to learn how to juice a lemon, not gaze at its delicious yellowness.
Grab a citrus juicer, a small strainer and the bowl or container you would like to store the lemon juice in. And your lemon(s), of course.
Stick the lemon in the microwave for 10 seconds to soften it.
Cut the lemon in half, directly through the center.
Place the lemon halves, one at a time, directly on top of the citrus juicer and press down while simultaneously twisting clockwise and counter-clockwise repeatedly. Do this until the juices have stopped running from the lemon.
I happen to have a fancy citrus juicer with a handy slotted silver plate that keeps the really big chunks and seeds from falling through to the bottom. If you don't have this, do not fret.
Place a small strainer over the top of the container in which you wish to store your lemon juice. Pour the contents of the citrus juicer through the strainer and into the bowl. Even with my fancy juicer, there are still seeds and pulp that I don't want mingling with my juice that the strainer is able to catch.
Repeat the process with each lemon that you need juiced. Then enjoy making lemonade or lemon chicken or lemon meringue pie, or whatever it is that you are juicing lemons for!
I occasionally run my juiceless lemon rinds through the garbage disposal in order to give my kitchen a fresh, lemony scent.