My advice on watching videos, is to watch, and anytime you see something "interesting", re-wind and figure out what lead up to it. See how it was set up, or what caused player X to know that player Y was going to do Z. Try to figure out what the players were thinking when they made a decision. Figure out the logic behind why they thought it would be a good decision at the time, and then ponder if it was actually a good decision/the best decision. If I remember seeing a player make a really good/interesting read I watch it continually to log into my head what the situation feels like. Im able to do spontaneously good reads against people I've never played I've created familiarity with what is the logical thing for my opponent to think.
If you can rationalize why players like Ally make ridiculously good decisions, it can help you internalize them (How well you can do this varys from person to person though). Experience is extremely important too, but as you play and watch videos your arsenal of tactics will expand naturally.