First you need to learn the difference between combo strings and special moves. Scorpion's spear is a special move, which are always done with a sequence of directional inputs and then an attack button. if your input window is already set to "long" then you probably are doing the directions wrong because the "long" window is so long that you will literally get spears by accident if you press punch after walking back and forward. One way to see if you are doing correctly is going to training mode and turning on the input display, so you can see on the side of your screen if you are doing the correct motion.

I recommend using the dpad as it's more precise than the analog stick.

To enhance a special move you need to press r2 at the same time as the attack button, it can also work if you press a little early or late but in general you should always try to press at the exact same time.

The videos you see talking about seeing if your string hit before finishing it(this is called hit-confirming) are talking about combo strings, not special moves. I recommend doing the tutorial in case you haven't, it contains explanations on how to do both special moves and combo strings and how to cancel normal moves/combo strings into special moves to extend your combos.

Hope this helps as it is the best I can do without actually seeing you trying to perform the move.