Or,,take a piece of card stock (like a business card thickness) and moisten it a bit on one side after rubbing a little transfer wax on the other side. Place the moistened side down onto the gun surface and with a burnisher, rub the card into the surface. The wax is to lubricate and avoid tearing up the card only. Has nothing to do with transfering the pattern to the card. After burnishing the card into the surface, carfully lift off. It will be firmly imprinted with all the finest detail of the engraving and markings and pits! All in reverse of course. Let it dry which takes a few minutes. The patterns are quite durable. To lay it back down, now take the transfer wax and dot it over the gun surface evenly and lightly. Need only show a haze of wax on the surface. Take the pattern and scuff accross it with a charcoal pencil to highlight it with the black. Lay it down into position and again very lightly with the burnisher go over it to imprint the black line pattern onto the wax coated surface. The pattern is reusable unlimited times if not abused. The edges and screw holes burnished into it help in lining it up when laying it back down onto the surface.
Takes 3 times as long to explain as to do. My transfer wax is now about 1/2 bowstring wax and 1/2 beeswax. I think there's a piece of crayon in there too. Kind of sick green looking!
Everyone has their own way of doing this. There is no 'best' way to do it IMHO. Excellent advise above is to let the engraver doing the work take his/her own patterns for the job. Anything else may be nice to look at but worthless to them in the actual engraving process.