For this you would need to:
1- Download the "character set" (tileset) from the Dwarf Fortress wiki.
    (a tileset is a png or bmp image contaning 256 tiles, corresponding to the 256 ASCII characters)
2- Place the image in data/art of your Dwarf Fortress file.
3- Open the file data/init/init.txt in the Dwarf Fortress file and modify it like this:

For fullscreen: 
[FULLSCREENX:800]
[FULLSCREENY:600]
[FULLFONT:montileset.bmp]

For windowed:
[WINDOWEDX:640]
[WINDOWEDY:300]
[FONT:montileset.bmp]

It is recommanded to keep the line [BLACK_SPACE:YES] for stability.
You can now save the file and play.
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