How to design Inset Lines in Photoshop
I have already explained one method to make inset lines in Photoshop.
Inset Line Effect in Photoshop
Here is another method to make inset lines in Photoshop.
Final Image of the Tutorial
Fill the layer with any color you with to have as background. I am using #3b3b3b color to fill the layer.
Set foreground color to white. Press and hold shift key and draw a straight line.
The first color should be little darker than background color, and the second color should be little lighter than background color.