Generally off the backing plates with two or three struts. Should be a handful of threads about this already maybe some one will link to them
Here are some https://www.jalopyjournal.com/forum/threads/cycle-fenders-how-to.276283/ https://www.jalopyjournal.com/forum/threads/cycle-fenders.1020738/
Build them stronger than you think you will need. Had to repair and reinforce mine. Once you watch your front end bouncing around out there on the open road you’ll see what I mean. Great info on the threads shared I wish I had in the beginning. Thank you!