Well,first,make the object you want to drag(A rock,for example)then create a number variable named dragged for the rock.
Left Mouse button clicked -"Rock"-set "dragged" to 1
"rock" isn't overlapping "rock" -
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Left mouse button is down -"Rock"-set position to "mouse"
"rock" dragged is 1 -
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On left button released -"Rock"-set "dragged" to 0
And there ya go,hope it helped :D