considering google and various help topics everywhere are being useless :/ ill post in the forums to see if i can get help here...

im trying to make a gif but have no clue what im doing... and all i want is a scrolling fog overlay on a screenshot that i have (i wonder why no one has ever covered this on youtube?) the fog is already a layer on the screenshot and i have an unedited screenshot in another tab, how do i make the fog and only the fog scroll over the original screenshot to make it look like a foggy night. at first i also wanted blicking stars but if scrolling overlays isnt there... then flickering wont be.

sorry if i seem impatient, its because i am. ive been researching for a few hours and been to a few chat rooms to only be utterly ignored... irritated and impatient :/