Take a look at the original sentence again:
Wally did not want to do anything to ruin his new pair of athletic
shoes, he always stayed on the sidewalks,
sidestepped puddles, and made several trips a day to the bathroom to dust
the sneakers with toilet paper.
You wanted to fix it this way:
Wally did not want to do anything to ruin his new pair of athletic
shoes, but he always stayed on the sidewalks,
sidestepped puddles, and made several trips a day to the bathroom to dust
the sneakers with toilet paper.
Using a comma and a coordinating conjunction like but is a legal way to connect two main clauses. You must be careful of one thing, however. The coordinating conjunction that you use must logically join the two clauses. But shows contrast and is thus inappropriate in the example above.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.