Example 1
Cody does magic shows on the weekend, most of his gigs are children's birthday parties.
Here we have an example of the dreaded comma splice. Let's make it disappear: Cody does magic shows on the weekend, and most of his gigs are children's birthday parties. See what we did there? Since there was already a comma, we just added the coordinating conjunction and, then… presto! A grammatically correct sentence magically appeared.
Example 2
Centipedes terrify Elsa, if she sees one, she'll faint.
We've been told by several people that centipedes are actually good to have around because they help control the population of the other pests in your home. We always tell those people, "Agh! It's a centipede! Kill it! Kill it!"
Now we'll tell you one way to exterminate this run-on: take out the comma after Elsa, and replace it with a semi-colon or a period. (If you choose the latter, don't forget to capitalize If!)