The way i do it is to, select the 'pen tool' in the toolbar, then create a path by drawing round the shape, then correcting the edges with the add anchor tool, which is again in the toolbar, once ive gone round and all the edges are smooth il right click and 'make selection' then copy and paste this into its own new layer then desaturate or greyscale the origanal layer. dont know if that makes any sense or will help you but thats how i do it