Don't know where you are going in Shelbyville so can't help you with the city streets there but I have gone there on my bike several times from home.
Two ways that are fairly direct.
Either way you can park at Christiana Element Sch
1. Head South on 231 and take a right on Hwy 269 (Walnut Grove here). Go Left on Midland Road and head straight into Shelbyville. Left at the last road takes you the light for the new Walmart on 231.
2. Head South a little more and turn left on Hwy 269. Continue through Bell Buckle and head out toward Wartrace. Turn Right on Fairfield Pike which goes more into the east side of town.
Both will be busier the closer you get to town but are better than 231. The first route is flatter. The second route has some hills. If I was doing this, I would consider coming in via route 1 and coming out route 2.
Invest in a new copy of the Tennessee Atlas and Gazetter. It has wonderful topo maps of all the back roads in TN. It is the map most of us use for club rides. I got mine at Sam's for about $11. Cover price is around $20. Most big convenince stores and Walmart have them too.