I've created a mat frame (white) and I'd like to place a drop shadow on all 4 sides. I assume this is a layer problem but I don't have the experience to figure it out. Thanks very much.
There's no such thing as a drop shadow appearing around all 4 sides of an object.
Create a new blank layer below your mat, and use the Rectangular Marquee tool to draw a selection slightly larger than your mat. Feather the selection to soften its edges and fill the selection with black or a gray.
Here's another way: (it's really up to you how you want the shadow to look)
1. Do a Select>All and put your frame on a separate layer by going to Edit>Stroke> Inside
2. Ctrl (cmd) click on the frame layer thumbnail in the layers panel for a selction of the frame
3. Make a new blank layer below the frame layer and fill with black; Edit>Fill
4. Select>Deselect and use Filter>Blur>Gaussian Blur (the example below uses 10 px)
5. Change the layers blend mode to multiply
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