You want to use wide heavy duty foil. It's width is sufficient-- if you cut a sheet exactly square, or a little larger-- to entirely line the standard WSM pan with about 3/4" to spare on the straight edges. What you don't want to do is poke a hole in the foil, or use narrower sheets seamed together-- then you will have the potential for the wicking your friend warns about. I line inside and out, some only line the inside, or the outside-- it's entirely personal preference. If you do line both, do the outside first, so the inside layer overlaps it. I change the outside foil only occasionally.
