Any advice on how to analyze markets?

[Bob Smith]

I have experience w holding rentals but I'm now just starting to go into buying strategy like brrrr.  My question is for any advice on how to analyze markets.  ex If search for things like best or worst markets to buy or rent etc. I'll find pages like this complete with heatmaps.  

How are they generating/aggregating this data?  

I'm perfectly fine with using data like this but I'd really like to understand and be able to gather and analyze markets myself but really have no concept yet of how to do that.

Any help/advice or a pointer to the right direction is appreciated. :)