I'd like to try it, but I have some problems.
I can never imagine something "pure". As in, trying to relax and think of a good shape for a tulpa, I see it in my mind, I even talk to it, and then, a spider leg comes the fuck out of it. Or the tulpa makes a scary face. Or the tulpa changes from good to scary, and then back, and so on. Or I somehow Know that it wants to hurt me. And then I try to picture a calm room, a white room or something "good", but then spiders come out from the ceiling, or shadows move around my room, or faces, like a bad horror movie, and the more I concentrate on "Make It Good Already!", the more shit it gets.
It's like I KNOW I should be thinking of something good, and some part of my mind says Fuck You. Well, fuck me then.