How do you do what?

Add a parchment texture to the background, or find a parchment texture?

I have a selection you can choose from - which I recommend you use at the size they are if you do. They are on the link in my signature.

Others also have parchment selections I'm sure they will volunteer in time, and you may find a few in the Mapping Elements forum

If you import the parchment as a new layer (File/Open as layers), you can then either replace the current background with it, or leave it on top of the layer stack and change the mode to overlay, or multiply.