I drew up a little chart/roadmap outlining all the diatonic chords of the key of C Major, with the addition of the Secondary Dominants and Tritone Substitutes. The premise of this particular chart is dealing with a I-vi-ii-V7 turnaround in numerous ways. Now, it is a limited group of examples that I've outlined, but in further posting I may get a little more adventurous with the examples. You can use the chart to help define your own choices (that's what I made it for). If you have any questions on how to interpret it, feel free to post them here and I'll answer the best I can.
For a quick explanation, you have the Tonic (C), Submediant (Am), Supertonic (Dm) and Dominant (G7) in the circle. Just outside of the circle in white lettering are: Mediant (Em), Subdominant (F), and Leading Tone (Bdim). The chords in red in a straight line with the diatonic chords are the Secondary Dominant chords. The way they are used is usually as a bridge to get from one diatonic chord to another, as they introduce a sense of urgency for resolution. Sometimes a secondary dominant will substitute for a diatonic chord, often in the case of a II7, III7 or VI7, particularly in turnarounds. The 3rd group of chords are Tritone substitutions, grouped accordingly with the chord they are a sub for (e.g., Bb7 is the bV7/iii--however, it is a direct substitution for the V/vi, or E7)*. So, essentially, you could replace E7 with Bb7 as you find your way to Am, or even better yet, A7 (for the sake of our turnaround.
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* To help you understand what the symbols V/vi, etc. mean, it reads "the 5 of 6", meaning the secondary dom of Am, or E7, easily figured by going up a fifth from Am.
Click on link below for larger image

Connecting Diatonic Chords using Secondary Dominants and Tritone Substitutes
I always have difficulty understanding what some folks write in trying to describe musical situations and theory. I apologize in advance if what I'm saying confuses you. I've tried to present it as clearly as I can, but for some reason it may not click. If you don't get it, give me a chance to help you get it.
--Mike

