How to create dropshadow on layer with alpha transparency in GIMP?

Question

I'm working on a project consisting of a background image with several layers on top of it. Each of the layers consists of an image with alpha transparency, such that only part of each layer is visible. How do I create a dropshadow around the visible portion of each layer?

I've tried following the instructions in the GIMP documentation, but the procedure described there never actually creates a dropshadow. So either I'm doing something wrong, or the instructions aren't meant for the situation I describe. Any suggestions?

Example: How to create a dropshadow from the black-and-white photograph (a layer with alpha transparency) that extends out over the green background?

Answer 1

The answer turned out to be blindingly simple: Just select the layer and then apply the filter. I did try that before asking, but for some unknown reason it didn't work at first. In the end, however, it did.