Step 2: Add a white Color Fill layer
In the Layer panel, click the Adjustments drop down and select “Solid Color”. Set the color to white (or whichever color mat you prefer) and click OK.
Alternately, you can use the menu to add a color fill layer by going to Layer > New Fill Layer > Solid Color.
Then, drag this new Color Fill layer so it is below the top-most image layer (either the merged layer, or, if you only have one layer in your document, move the Color Fill layer to the very bottom).
Tip: If you’re not sure what color you want to use right now, that’s okay! Because you are using a Color Fill layer, the color can be changed at any time. Simply double-click the color swatch in the layer to open the color picker window.