Serving technique in tennis covers grip mechanics, racquet positioning, and footwork that separate power serves from control placements reviewed here.
Here is a claim that sounds wrong: the fault is not the thing costing you the match. A single serve landing in the net or long is worth nothing on its own. It is neutral.
You have stood at the baseline, down a break point, and asked yourself some version of this: what am I actually allowed to do here, and which serve gives me the best odds?
The advice you have probably heard goes like this: if the ball clips the net on a serve and still lands in, you call "let" and serve again. Clean, tidy, done.
Watch a busy public court for an hour and you will see dozens of serves that break the same rule. Almost none get called.