She does not take a follower slot.

Extract the files in the Fallout 4 directory, so that the txt files are in the folder with Fallout4.exe and the esp is in the Data folder.

When in game, go to Darla at the Vault 111 entrance and click on her in console then type:

"bat darla"   (be sure to spell it right)





To get her to stop being a follower just uninstall the mod by removing the esp from your load order.