You place a *.lyt file in your LFS/data/
layout folder. Then when in the game, if you've loaded up the right track that the layout is made for, you press SHIFT + U, and click load layout (press space to show/hide editor menus).
So if you downloaded
BL3_layoutnamehere.lyt, that would be a layout file for Blackwood Carpark (BL3).
This can be done in Singleplayer and in Multiplayer if YOU are the host. If you are connected to a dedicated server, you cannot use the editor on your own computer, you need to load up the layout in the pre-race screen.
Here is more about the Autocross Editor in the LFS Manual ---
http://www.lfsmanual.net/wiki/ ... Editor_Instruction_Manual