I don't remember where I read about it originally, but here's my personal meditation practice for imposition.

Sit down in a calm and quiet environment, during a time period where you won't be interrupted. Get comfortable and close your eyes. Take some deep breathes and try to clear your head. If you have difficulty doing this, grab and light a candle, set it in front of you and focus on the flame. Drive all of your thoughts out of your head as you focus on the gentle flame.

Once you've cleared your head and are calm and relaxed, shift your attention to your forehead. Focus on the center of your forehead, imagine shifting your energy directly to that spot. You should feel a small tingly feeling after a bit, if not, don't worry just keep focusing, or move on to the next part.

Now, imagine there is a mouth on that spot, right in the center of your forehead, with it's lips closed. Try to visualize it as best you can, while still trying to keep that energy pinpointed on that spot. Imagine the mouth opening, and taking a breath. Whenever it breathes, the energy getting just a bit more intense. Try to keep a steady rhythm of breathing going, energy pulsing with each breath.

Once you've gotten a decent rhythm down, open your eyes and try to keep the sensation of the mouth breathing while you're eyes are open.
Keep practicing this until you are able to feel the mouth breathing even while fully concious and alert. Always trying to visualize exactly what the mouth looks like.

A slight re-skin of this method that may be more appropriate to hear if you use unicorn files, would be to invision your horn instead, and instead of breathing, try to channel magic through your horn.

If you start to get a headache, or feel exhausted, feel free to take a break and come back to it later.

This excersize works by bringing more blood to the area of the brain responsible for imposition, essentially overclocking it and making it easier to feel foreign sensations, and eventually impose objects in your vision.

I can't vouch for any science behind this, all I can say is this is the method that I've used, and it's worked for me