this is something i literally tried yesterday.
i was going through a book on hermetic magic and in every ritual they say to draw the sigils of certain spirits over the pentagrams. (this is a highly repugnant practice that steals energy from the magician and gives it up to the spirit, i dont recomend it) so i decided i would use this technique to summon a spirit. i chose the demon Azazel because his sigil is fresh in my memory. i drew the sigil in mid air in front of me, the color of white light, and let it hang there while i called for Azazel vocally. holy shit did he show up fast!
full disclosure: i wasnt in a circle i was just sitting in a chair. this was purely an astral evocation, no incense or black mirrors, but the spiritual presence was so strong i could not deny my success.
yeah guys, try it out. see how it works for you