Sit comfortably and hold your instrument at about waist level. The guitar should ideally fall at about your waist, although more experienced players adjust the height to suit their taste. A beginning player should be sure they are as comfortable as possible to aid in learning to use the thumb.
Form a chord on your instrument. The thumb strum can be accomplished when you aren't playing a chord, but for the purpose of learning, a simple chord will make it easier for you to hear how strumming with the thumb works and to see how it is accomplished.
Pick some of the higher strings with your first and second finger. These are the thinnest strings. Sweep them up toward you as you pick the notes. Pick two or three notes one after the other, then immediately roll your thumb across all of the strings, moving away from you, from lowest to highest. This is the basic thumb strum. As you advance, you can begin bringing your thumb back across the strings in the opposite direction or strumming only one or two strings.