Sometimes, I'll have several layers on an image and I want to crop one of them. NOT resize it, crop it. Every technique I've tried simply crops EVERYTHING, not just the layer I'm working on. I have to delete the layer, crop the original image that it's from, then re-paste it into what I'm working on. I'd also like know how to rotate a layer without rotating everything else in the entire image.
Let us assume you had selected a layer as an active layer which can easily be done using the layers menu:
This makes the ants yellow to indicate the active layer
If you then want to crop just this active layer make sure you had ticked the "current layer only" box in the crop tool menu
Now, when you apply the crop tool a new active layer will be created as cropped from the previous active layer:
To crop a selection from a layer choose Layer > Crop to selection from the top menu.