These measures make use of basic rasgueado with the right-hand index finger. The finger will begin with a down strum and will alternate down and up strums to the end of the passage. To stabilize the hand, rest the right-hand thumb on the sixth string.
The strumming motion should be in the finger--don't move your arm as you play this passage.
Keep the other fingers relaxed: it's ok if they move somewhat in sympathy with the index finger, but be sure only the index finger strums the strings.
Try to hit all five strings on the down strums. On the upstrums it's ok to only hit the 3 or 4 highest sounding strings (even though five notes are shown in the notation).
The same passage occurs again in measures 13-16 and 25-28.
These measures make use of alternating rest strokes with the right-hand index and middle fingers. Rest strokes can be very powerful and are often used by classical and flamenco guitarists when playing one note at a time. Rest the right-hand thumb on the sixth string while playing the rest srokes on the first string. In addition to stabilizing the hand, this will prepare the thumb for playing the sixth string in measures 6, 8 and 12.
If rest strokes are new to you, begin by placing the center of the fingertip of your right-hand middle finger on the first string. Gently press into the string. When you play a rest stroke on the first string, you press into the string, and the act of plucking the string causes the finger to land on the second string.
Do this repeatedly with the middle finger. Then try doing the same with your index finger.
After you are comfortable doing a rest stroke with each finger, try alternating the fingers.
In measures 5-12 of First Spanish Study you will play alternating rest strokes. Watch the right hand carefully to be sure that each note is played with a rest stroke. Also be sure that you are alternating your fingers. If you start with the middle finger, your middle finger will play on each beat, and your index finger will play between the beats.
These measures are played entirely with the right-hand thumb. When the thumb is playing single notes, you can stabilize your right hand by planting the middle and index fingers on the first and second strings. These notes will sound more powerful if you play rest stroke with the thumb.
To play a rest stroke with the right-hand thumb, place the thumb on the fifth string and press into the string so that the plucking motion causes the thumb to land on the fourth string.
When you play the chords in measures 18, 20 and 24, forcefully strum with the thumb. The index and middle fingers obviously cannot be planted at this time, but can be replanted when the single notes resume.