Classical guitar finger picking is a technique that can be applied to many other styles of guitar playing. You can just wing it and figure this stuff out for yourself, but you are bound to pick up "bad" habits (there is perhaps no such thing as a bad habit, only your own unique style, depending on your point of view).
The classical style is logical and, with practice, will help you to easily find the strings you are looking for.
A good starting point is to finger any chord shape with your left hand (perhaps start with an A, but it's entirely up to you). The general rules of finger picking (which will always get bent or broken, they are more like guidelines!) are as follows:
The thumb is generally used for playing the top three (bass) strings, 6, 5 and 4. Fingers 1, 2 and 3 are generally used for plucking strings 3, 2 and 1 respectively. A good exercise to practice this is as follows:
1. Place your plucking-hand thumb on the top string and your index finger on the 3rd string. Place your 2nd finger on string 2 and your ring finger on string 1. This is your starting position.
2. Pluck the top string with your thumb, allowing it to fall onto the 5th string. Then play the bottom 3 strings with your three fingers, one at a time, starting with your first finger on the third string.
3. Repeat step 2 above, but play the 5th string with your thumb, allowing it to fall and rest on the 4th string.
4. Repeat step 2 above, but play the 4th string with your thumb.
5. Repeat this whole process over and over, picking and choosing between which of the top three strings your thumb plays.
Notes on the above:
1.Your right hand (if you're playing right-handed) should be roughly over the sound hole.
2. Try and play the strings so that you pull them upwards or downwards towards the adjacent string, rather than out and away from the guitar.
3. Fingernails definitely help to get a nice ring from the string. If you have fingernails, then the actual technique is to play the string with your finger, and the nail catches the string on it's way passed. But you can introduce more volume by using more of the nail and plucking harder.
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