If LFS is installed with all file associations,then by just double clicking (opening) the .lyt file. If not - you have to save it manually in lfs\data\layouts
To use a layout,load the track layout is built in (layout name starts with track code like AU1) and pick one of the layouts shown in menu. Or press shift+U while being on track in single player and click on "load layout" in lower right corner. Should be enough info,I believe.