Copy the files on the Left. Rename the copies of these as the files on the right:
xdgc.pcx -> lxdgc.pcx
xdgp.pcx -> lxdgp.pcx
xdpc.pcx -> lxdpc.pcx
xggc.pcx -> lxggc.pcx
xpgc.pcx -> lxpgc.pcx
xtgc.pcx -> lxpgc.pcx
wcso.pcx -> lwcso.pcx
wooo.pcx -> lwooo.pcx
wsss.pcx -> lwsss.pcx
Note that upper/lower case is irrelevant.
Explanation:
Basically, for Base Terrain Types (Plains, Deserts, Grassland, Tundra, Coast, Sea, and Ocean), the graphics are taken based on which terrain types are next to each other (allowing transition effects).
The terrain types beginning with x (e.g. XTGC.pcx) are predominantly, and in some cases entirely, land based. The terrain types beginning with w (e.g. wOOO.pcx) are entirely water.
Each letter in these files represents a terrain type:
d = Desert (Floodplains also uses this).
p = Plains
g = Grassland
t = Tundra
c = Coasts (note how c is in both water (w) and x (land) files)
s = Sea
o = Ocean
So wOOO.pcx is a terrain file full of only Ocean tiles which transition with other Ocean tiles.
But wCSO.pcx contains transition effects between Coast, Sea and Ocean tiles.
Note that some combinations aren't possible - there is no capacity for a file interacting between Tundra and Desert tiles or Tundra and Plains tiles.
The files work by tessellating the graphics in these files with the corners of terrain on a map. They are the same size as a tile but instead they take up 1/4 of the North, South, East and West tile of a corner. This is why in the editor you can't have a 4-tile diamond of (clockwise, E-W) grassland, desert, plains, coasts.
So why rename the files as shown?
Basically, because Landmark terrain was only introduced in Conquests, terrain packs in Vanilla and PtW won't have LM terrain. For base terrain, the simple answer is to copy the files you're using for non-LM terrain, and add the L prefix - LX is basically Landmark Land and LW is basically Landmark Water.
Forests/Hills/Mountains are a little different, but if you don't add LM graphics it won't mess things up too much (it'll just use the standard C3C graphics).