Dozens of ways, depending on the nature and quality of the original. The
cleaner and crisper the original, the better any of them will work.
Some places to start:
make a copy of your photo, increase the contrast to the point where it is a
little too intense (if you were going to leave it like that) then (in
--open HSL put a check in the colorize box and set the saturation to 0 (this
converts your image to shades of gray without leaving 24 bit mode.)
Try any/all of the following (which one is best will depend on your
--- adjust the threshold until all the grays drop out and you have a pure
black and white outline- this one is least likely to work unless you have a
very crisp photo)
-- run the pencil or black pencil filters
-- run the find edges adjustment (you nay have to run a negative image
command to get black on white instead of white on black )
-- run posterize, then find edges, run negative image, then blur, then
-- make a copy of your background layer, run a negative image adjustment on
the top layer, change the layer blend mode to dodge (your screen will go
white) then Gaussian blur the top layer gradually (turn your preview on)
until the outline appears
If these don't simplify things enough, try running the posterize effect
after you convert to shades of gray but before you do the others.
There are a number of plugins (some free, some not) that can speed the
process up. The best, IMO, comes from Segmentis, but it isn't inexpensive.