Ah yeah, imposition. Something sought for not only by tulpamancers but ponies doing hypnosis as well.
Imposition is by far the hardest part. Some get lucky while others take a very /very/ long time. Myself, I can visualize a bit what I/my tulpa look like though I can't exactly impose that into my sight.
I used to get very much depressed when I couldn't see my tulpa, though I got over it once I realized that I don't need to actually see them to prove their existence. Imposition is nice, though not required. If I do end up seeing them, then I guess that's great! But we're all just as happy at the stage I'm currently at right now.