Watch the Western Swing Solo online guitar lesson by Jason Loughlin from Essentials: Country Soloing Styles
There are three approaches I'm using when playing over the G chord: I'm connecting and embellishing chord tones, outlining arpeggios, and using the G major scale or major pentatonic scale as passing tones. For the rest of the changes, I'm not really using any scales, I'm just outlining and embellishing chord tones. The first substitution is a G augmented leading into the IV chord. For the raised ivdim, I'm just outlining the arpeggio and chromatically connecting the 3rd to the 5th of the G chord. The rest of the cycle of secondary dominants is almost all arpeggio notes.
The second chorus of the solo uses a descending scale pattern down a G major scale. Our next substitution uses an Ab diminished arpeggio to access the b9, b7, 5th, and 3rd of our G chord. This leads nicely into the IV chord. We use a substitution again over the E7. We're going to split the bar into two beats of Bm and two beat of E7. This is called a ii-V. Bm being the ii of the next chord A and E7 being the V of A.
Leading from the D7 back to the I chord, we imply the tri-tone substitution. A tritone sub is a dominant chord three whole steps away from the root of the V chord. In this case, that would make the sub an Ab7 chord. The Ab and Eb at the end of our solo come from the Ab7 chord.