Help, just can't remeber how to adjust layers...

Getting old or...

If i select a part of a picture and then make it a new adjustment layer (levels, curves whatever) then i can adjust it as i want - fine, however, how do i adjust one layer only without affectiong the layer below?? Totally lost here

Lets say i make a selection of background layer and put it as a new layer with Command J, how can i now adjust this layer without affecting bottom layer...

Feel really stupid now as i must have done this thousand times already

Cheers in advance

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