If you've ever played Skyrim, you'll recognize that it works a bit similar
With your arrowkeys you use the lockpick.
- Up/Down keys are for moving the lockpick left and right.
- Left/Right keys are for picking the lock
(Why isn't left/right for moving the pick left/right... who knows
There is, as far as i know, no way to know if the pick has the right angle. It is a matter of trial and error.
I suggest you stock up on lockpicks (vendor is in Slint... all the way down near the docks) and lightly tap the right/left key when you pick the lock. It doesn't move? Angle the pick a bit different and tap again.... and so on untill you get it.
The problem you'll have now is that as soon as the pick breaks, you'll exit the little minigame and upon trying again, the 'correct position' has changed.
These positions are completely random and as mentioned above, there is no way to know where the correct position is.
My advice for Lockpicking:
Either ignore it as the rewards are generally not thát useful. some are, but the majority isn't.
or first level up lockpicking to the max and then lockpick away.
I hope this helped.
(PS: Lockpicking isn't just for Thief though)