Focusing on a sigil in your mind’s eye will bring a mental form of that sigil into your immediate environment, so that you can use it in your practice to act out your desires. This can be useful so that you will be able to use sigils even when you do not have tools to make it physically.
This would be like focusing on a sigil for good luck in your head, so you can invoke good luck to you. A practitioner could also focus upon a sigil, and somebody else in the immediate vicinity in order to connect them to their will, and cast it upon them, so that they will be affected by their desire. To do this you will need focus, concentration, and willpower, so that you will be able to envision, charge, and activate mental sigils to affect yourself, and the world around you.
In order to do this all you need to do is envision the sigil in your mind as clearly as you possibly can, then focus on that sigil in your mind while you internalized its image. Once you feel that you have done this for long enough, visualize the sigil being destroyed, so that it will be activated, and begin manifesting your desire into the world.