Hello Bigguy Here with Tabletop Bits And Bobs...

I have hand drawn maps for commissions for a few years now and this is a trick that I use to make my maps look old.

I use a parchment texture. It's put on last in the digital layers and placed on top of everything else.

I found that putting it at 40% opacity is a good number to not only fade out the drawing making it look ancient, but automatically give the map a sense of legitimacy that is difficult with stark white maps.
I made the texture by combining all kinds of browns and tans and filtering it until it looked correct. You are free to use this texture and my trick to add a little dash to your future maps.

ParchmentTexture.png

Here is the texture at work:

#1_Continent[map,handdrawn,collection,continent,custom].jpg

If you wish to see more maps that I have done with this trick go to my Website and scroll down to the Example Gallery.