How to know when to buy? (VOO, SPY, QQQ, VTI)

I'm new to investing, I did my research and those stocks seem like the most stable investments (good yearly returns) I would like to start out with. I would like to follow the rule "buy low, sell high". What is a good indicator or rule of thumb when trying to buy at the dip? 1-5% dip?

Some may say "the best time to plant a tree was 10 years ago, the 2nd best time is now". I can wait a few weeks for a dip, I am in no rush but I would like to know some indicators when I should get in at a fairly safe dip, I don't want to end up waiting for the dip to go further and have it come back up.