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Grab the pen tool and trace out the shape where you want your line to be.
View attachment 7988
Take not of the paths option
View attachment 7989
After you have finished tracing it out double click the work path in the pallet menu.
View attachment 7990
select the saved work path and press cntrl+t to transform the path. Holding Alt+shift, click the corner of the transform box and move the path out from the edge enough to not cut off some of the image. When done you will notice it will look like this but all the way around.
View attachment 7992
Now get the pencil tool that is under the paintbrush tool. Select a 5 pixel brush
Go back to the paths pallet and click the stroke with brush option.
View attachment 7993
If done properly you should end up with something like this.
View attachment 7994
Hope this helps and good luck.
Totally lost in PS, after I do that. How do I select the nodes to fine tune the shape. Thanks