What is your favorite color? That color resembles you, every time you think of that color you need to think of yourself. Now have your tulpa choose a color, it can't be close or the same to yours, if they do choose a color close or the same as yours, either you need a new color or they do, but just make everyone happy (I recommend blue and red).
Now associate their color with them, if you think about that color you should think of them. Now, onto the possession part, rest your hand on something, get it comfy, I don't care how. Look at it in your mindscape (thats the term I use, meaning how your mind's eye sees the real world, look at the real world through your mind's eye, you can close your eyes if you need.) your arm will be your color.
You need to drain your hand of the color until you almost can't feel it, disassociate yourself from your hand. Drain the color into somewhere in your brain or something, in one very small point. Now ask your tulpa "Do you want to do this?" and "Are you ready?", you don't want to force them into a possession.
If they say "Yes." then fill your hand with their color very slowly, at first all that will happen is slight twitching, ask them to touch your thumb and first finger together, at first the movements may feel like you, try to avoid this feeling.
Once they got that down, ask them to clench your hand into a fist, the movements will be slow and choppy at first, but they will soon learn how to work your body. Do this as much as you can, then, when you feel ready, start moving up your arm, the first hardpoint is your upper arm, your tulpa must possess all the muscles that control your arm, and there are a lot of back muscles that you don't even realize you have. This method works for your whole body, but when you do this, you must do it for EVERY PART, your eyes, ears, mouth, nose, lungs, throat, voice box, everything. It takes a long time for full control to be accomplished, but when it is, the feeling is amazing.
Now that your tulpa can move your hand, have them try and put it in a fist, while (not taking control) try to open in through your subc, it will become a healthy battle for the hand, if they close it they win, if you open it you win, simple as that.
Once Liquid color becomes easy to you, you can do what I do for possession. Disassociate from your body completely, then, starting at the top of your brainstem, create a supernova of your tulpa's color, have it (very slowly) go over your whole body, the more you do this, the faster the supernova can go.