All you need is Heavy Whipping Cream, salt and a couple of baby food jars.
Pour the cream into clean, dry, baby food jars (or other small jars with a tight lids).
Screw the lids on tightly and shake shake shake your jar. Within about 5 minutes of constant shaking you will notice the cream getting thick and a liquid forming around the ball of butter.
That liquid is actually buttermilk. Pour it off (and save for another use if you like, or discard) and screw the lids on tightly again. Continue shaking for about 5 to 10 minutes longer. You'll know it's butter when it looks like butter!
Add salt to taste and spread on some fresh, homemade bread....