I produced a few articles on general S/R theory a couple of years ago.
Plus an article demonstrating the trade opportunity available when S/R changes polarity (S becomes R):
And a recent blog post which demonstrates how I define my S/R levels on my higher timeframe: “Plotting S/R – Here Are My Levels From Today’s Market!”. (NB. You’ll find this blog post in the YTC Newsletter signup bonus ebook, “The LOST Files – 150 Lost YTC Blog Posts”)
At the very least, please read the recent blog post prior to this article.
Now, in writing all of the above, one thing I don’t appear to have ever discussed is when I will invalidate a level and remove it from my chart.