Sounds like you are determined to figure this out, so I am sure that you will get the hang of it.
I may be repeating some advice here:
Walking Foot is for Straight Lines...
Stippling or Meandering is what you were talking about when creating the wavy lines or turning curves, etc.
To do straight lines, you leave the feed dogs up.
To do Free Motion Quilting, you drop the Feed Dogs or cover them and You control the length of the stitches by moving the fabric with your hands. Free Motion needs to be practiced on a sandwich of scrap fabric with batting and backing to get the hang of it. Use a Darning Foot for free motion quilting. That's the basics of machine quilting. The thread that you use and the needle, etc. will affect the tension especially of the bobbin which can sometimes be a problem.
Hope this helps. Am still learning my self.