noteheads means muted open strings, sign means downstroke, means upstrokeTry to play this eighth-note rhythm for a bit until you feel that the rhythm is stable even when you do not think about the strumming hand at all. Keep it, say, 2 to 5 minutes, and try playing with the metronome at 60 bps playing two strokes/eights per click. Practicing with the metronome is very useful because it teaches you to play clear and precise rhythm which allows playing with anyone without any obstacle and to be rhythmically precise without any thinking about it. The next step is to emphasize each first of the two downstrokes (emphasize it with a stronger stroke):
sign means an accentThis is a basic strumming pattern around which we may build many variations but one must be very stable in performing it in its elementary form. How are we going to vary it? Simply, continue to repeat the previous basic rhythm pattern and when you feel ready, try to throw out one upstroke. E.g. you may try not to play every second eighth in each bar but say "ta" or "da" instead of that-one kick in the muted strings, like this: The up-and-down movement of the right-hand may be continuous and uninterrupted even during the pauses by moving the right-hand just beside the strings, saying "ta" or "da" at the same time. That's the meaning of the up- and down-strokes in parenthesis. This permanent hand movement is very important for the skill of so-called organic strumming. After some time you may throw out the second eight in the second group of four eighths as well: Or you also may throw out the first eight in the second group of the four eighths, and you will get another one rhythm pattern: And so on. Be creative and try to mix these few patterns and you even will be able to invent many more, but remember – first need to learn separately the most basic pattern and the next few which have just been shown. Only then you will be able to improvise and play combining them organically. Now we may also include the left hand and try strumming over a single chord, and in the next step, over a change of two or more chords as well. I suggest to start by using a simple chord change like is the change between A minor and C major chords because it is the one of the most simple to learn. Let's see how the simplest transition between these two chords works. The third finger is the only one that moves, and by changing its position from the fifth-string/third fret to the fourth-string/second fret you will be able right away to make a very swift and smooth change of the two chords. If you incorporate these two simple exercises into your daily playing you will very quickly feel a significant improvement of your strumming and chord changes skill. And if you want more lessons like this to improve your skill very easily but at the same time in a thorough and systematic way, I suggest taking a look at Acoustic Guitar Course by Steve Stine as one of the best courses in its area. There you may find more about essentials like Guitar Tuning, Holding the Guitar and Guitar Picks, then about Power Chords, Open Chords, Barre Chords, Bouncing, to more advanced skills like Beat and Subdivision, Kick-and-Snare-Strumming, Scales and Alternate Picking as well as to learn some of the most iconic acoustic guitar songs like Pretty Woman by Roy Orbinson, House of the Rising Sun by The Animals, or Like a Rolling Stone by Bob Dylan and Layla by Eric Clapton, and much more.