Mental sigils are sigils that you send out through your mind. You are able to do this with any form of sigil from once you create yourself, to others that you may find. Mental sigils do work, and to use them you would only have to imagine a sigil in your mind, and focus upon it to charge it with your power. This charging process will usually be more effective if you allow yourself to be able to focus on the mental representation of the sigil while in an altered state of consciousness.
Once you have that sigil charged in your mind you could activated in the same two ways you would other sigils through the understandings of passive and active activation. Allow it to exist, and release its power over time, or destroy it in your mind, and send out it’s power all at once. To destroy it all you need to do is focus on it being destroyed, and no longer existing, while visualizing that image in your mind’s eye. Even if you remember the sigil after you destroy it, that is okay just know that it is destroyed, and that its power has been sent out to manifest your desire.
Mental sigils can be very effective to incorporate into your practice especially when you do not have utensils to make physical representations of sigils.