I like that you mentioned how it doesn't have to be perfect to be a visualization. Even a simple doodle/step above a stick figure can help. Example;
Even if you have no knowledge of anatomy, you can jot down basic facts like... What color eyes? Any wings? How tall, compared to you? How long their hair? Et cetera.
If you have a lot of color-materials like pencils, or--for endless colors--a computer program with color sliders, visualizing colors can help bring physical forms to life. Even if you're totally colorblind, you can probably tell if a color is darker or lighter than another.
If you don't know where to start, draw yourself first, and then draw your Tulpa next to you, now that you have a base to compare your differences to.
Like g0atblush said, feel free to redraw often as you both develop. It's a fun way to show how far you've come.
--Mikhael & Johannes