personally I use an electric trimmer but the classic way would be the way barbers did it may years ago and that would be with a straight razor. I remember as a kid in the 1950s going to the local barber shop and the barber would dispense some shaving cream from his lather king put it on my sideburns and strop his razor and trim with it. I imagine it was not the same as the straights he used for giving shaves only because actual hair trimming can be rougher on a straight than normal shaving.