It might be surprising, but until this point it's been pretty much impossible to import a custom map into the original Dark Souls. According to Souls modders, the closest you could get was importing maps from Demon's Souls - and while modders have worked on custom maps for some time, problems with the specific file format used for collision detection in Dark Souls 1 meant that progress was stalled.
Until now, that is, as several modders have figured out a way to import working custom maps into the game.
As explained by modder Zullie, progress began in earnest when modder Meowmaritus created a tool to import models, but without collision detection (in layman's terms, the physics required to make things actually solid). The next step was carried out by modder Horkrux, who recently developed a way to create collision maps.