its simple work, but a lot of it. in masterworkdwarffortress/tilesetname you find a set of files, data/art, data/init, raw/graphics and raw/objects. in raw objects you have all the raws in which tile-numbers must be changed.
So you take one of the sets I already include, open the raws, and change all tilenumbers, which look like [TILE:x] or [ITEM_SYMBOL:x], to whatever fits the tileset. For this you need the originals from the tileset, so you first download mayday raws, and then countercheck the tile numbers with the tile numbers for the same entries in the modded raws, easiest done with win-merge or beyond-compare.
Thats it. The rest I have to do in the GUI sourcecode, adding the new tileset to it.