Create a scale chart that includes every major key that you typically play in your sets. Write the note names in a horizontal rows. For example, the key of G would be as follows: G A B C D E F#. Notice that the last note is not written. It has the same name as the first note, therefore there is no need to name it again. Above each column of notes, write the numbers 1 to 7. For a table listing the most common keys used for guitar, refer to "Nashville Numbering: The Gospel Music Archive -- Tablature and Chords for Guitarists," in the Resources.
Write the appropriate number above the chord names on a lead sheet. For example, a lead sheet that has the chords G G D G G C D G would now have the numbers 1 1 5 1 1 4 5 1 written above each chord name.
Write a new lead sheet using only the Nashville numbers. Transpose your song mentally to the key of C and play it. If you used the above example, the chords would now be C C G C C F G C. Play your song in the key of F. Continue playing the song in different keys using the numbers to determine which chords to play. Once you have memorized your scales, you will be able to quickly transpose chords of songs written using the number system into any of the keys you use.
Develop a deeper understanding of music theory by studying how different chords are made from notes in the scale. There are many books and online resources available that cover this topic in depth. Briefly, in any major key, 1 is a major chord, 2 is a minor chord, 3 is a minor chord, 4 is a major chord, 5 is a major chord, 6 is a minor chord, and 7 is a minor diminished chord. Letters, numbers, and symbols next to chord numbers tell you what quality of chord is to be played. Examples include major, minor, 7th, 9th, augmented and diminished.