First, you need to download those layouts (to LFS/data/layout folder). You can do that by:
a) downloading them from
this thread or
b) downloading them when you are on the server and those particular tracks are on. Press SHIFT + U, you'll have the menu where on bottom right is an option to save the layout.
Second, go to single player mode (NOT hotlap mode), choose Aston, for example. Press CTRL and you'll have "X" and "Y" options next to each track. Press "X" on Aston Cadet, for example, and the list of layouts on Aston Cadet will appear bellow. Simply choose the one you want and that's it
Set what you want: quali, race with only you or race with AIs.