Originally posted by xLew
Simply find a quiet place where you won't be disturbed, You could use your apartment or garage for this but i used a quite place outside around the back of my place.
What you will have to do is enter stealth mode by pressing the R3 button (Your character will slouch a little), Then using a rubber band or something similar to hold the analogue sticks facing each other, This will make you man walk in circles quickly increasing stealth. I think every 47 yards adds 1% or so the guide says.
I am doing this outside de to the fact you can equip a weapon, If you have a silenced weapon equip it, Im not sure this will increase it any more but who knows.