Single Finger Strumming
1 - Place the right hand thumb on the sixth (lowest note) string.
2 - Strum down, that is from low note strings to high note strings, with the index finger ("i").
3 - Strum up, that is from high note strings to low note strings, with index finger.
4 - Repeat the down and up strumming. Keep the thumb on the sixth string for stability. The finger that is strumming should move from the knuckle, the hand should be still. Vary how many strings are being played: all strings, just the highest three, the middle three, etc.
5 - You can time the exercise, e.g. strum for one minute, two minutes, five minutes, etc. Or, you can play a measure of sixteenth notes or a four bar phrase. The most common unit is one beat, or four sixteenth notes. That is: down, up, down, up. One measure would contain four beats, or sixteen strums. A four bar phrase would be four times sixteen or 64 strums. Don't try to count, just feel the groupings. If the sound is driving you and/or your loved ones crazy, mute the strings with the left hand or place a finger across the strings at a fret with a loud harmonic (e.g. 12th or 7th)
6 - For best results, keep going until you feel definite fatigue in the finger and hand. If you don't make the finger work harder than it's used to, it won't get stronger.
7 - Repeat with the middle ("m"), ring ("a"), and little ("e") fingers.
8 - For the thumb, place the "e" and "a" fingers on the soundboard below the strings. The "i" finger may rest on the first (highest note) string. This is the flamenco "thumb position" used in passages where the thumb is doing most of the playing. When it comes to thumb technique, flamenco guitarists can blow any classical player out of the water without even breaking a sweat, so it is worth investigating what they know and how they play. Strum down and up with the thumb, as above.