Comps and ARV?

[Wane Tango]

How do I learn how to become good at comps and/or ARVs? I have been told over and over how REIs make (or loose) money at the time of purchase. I am unsure how to become good at this, find more information, and how to practice. 

I am still fairly new, but I have taken my first steps at becoming a part time investor. I have parameters for a mailing list, and the start to a buyers list (currently 2 but they are people that I know). 

I have also taken the time to set up an appointment with several lenders in my area so I will know what my potential buying power is on day one. I feel like I have a good grasp of how the process will flow, I can see the ins and outs, the beginning middle and end, but the one thing the seems like could totally make or break a deal is the one thing that I know the least about. How to identify a good deal from a bad one. 

Any and all advice is appreciated.