Since you probably don't have a churn handy (and if you do, you probably already know how to make butter), this is a recipe for making butter in your blender.
Take a quart of fresh cream (or, if you don't happen to have a dairy cow, buy a quart of heavy whipping cream from the store) and let it sit at room temperature until it reaches about 55 degrees.
Put it in your blender or mixer, and turn the speed down to the slowest it can go. This is important! You'll have whipped cream if you go too fast!
Eventually (anywhere from 10 minutes to 3 hours, depending upon the cream and the temperature of the cream) the butter will "break" and you will hear a sloshing in your mixing bowl. If you look you will see tiny grains (or larger blobs) of what looks like it could possibly butter. The rest of the liquid is called "buttermilk" (not the cultured kind you buy from the store). Strain it away from the butter, keeping the butter in the bowl. Save the buttermilk for baking or drinking. It's just de-fatted milk.
Put a little water in your bowl and mix it again with your mixer or blender. This is "washing" the butter. Drain the water off. Do this several times until the water drains clear (not cloudy).
Now you need to "work" the butter, to get out any remaining buttermilk. A wooden bowl which has been soaked in cold water is best. That's because the butter won't stick to the sides. When I first started I just used a plate. Use a wooden spoon or butter paddle (that has also been soaked in cold water) and (for lack of a better term at the moment) squish the butter/scrape it into a blob...squish the butter/scrape it into a blob... over and over until no more buttermilk squeezes out.
Now you have butter. If you have a butter mold, you can make a fancy block of butter. I usually put mine in a small butter crock. Before I had a butter crock I just wrapped it in plastic wrap and put it in the fridge.
You gotta try it just once!