I learned this technique about 7 years ago in my first class with Rik Pierce and Sometimes I use it when I am completely out of miniature flooring (like I am now). 1st you take a Peel & Stick Vinyl floor tile and cut it to fit the room you wish to install the finished floor in.
Then you check to be sure it fits good BEFORE you start the next step. Once you are sure it will fit, take the piece out and take your pencil and start drawing where you would want your grout lines to be at. You can also purchase a copy of Rik's "How too" in Dollhouse Miniatures Magazine 12/05 Issue (Which I can't find now LOL).
You would then take a straight edge hobby knife to cut the grout lines. Be sure you have a SHARP edge on your knife!!! Dull knives causes injuries! You would cut each side of your pencil line and then lift up that strip to reveal the "grout" color underneath.
To adhere it to the floor, take off the paper backing (where it's sticky) and add some hot glue around it and then press it on your floor. This is the finished product.
I have used this technique in several different Dollhouse projects now. It is a nice way to create that custom look to your dollhouse miniatures!