My dear Jane,
Thank you for your letter about your practice of shamatha. I rejoice. It is very good to try. See yourself as the deity, so it becomes tantric practice as well as shi-nä.
To develop shi-nä, first go through all the details as mentioned in the sadhana. Then, if the deity has a third eye, try to concentrate and focus your mind on that. Later, on the basis of that, without losing it, add the details of the face. If that is clear, without losing it, add the simple aspect of the deity: two hands and legs. If this is clear, expand the arms and legs and after that, when you are clear as the deity, add the mandala. This is one way.
Another way, according to Lama Tsongkhapa, is to focus on yourself as the deity with one face and two arms. After going through this, focus on whatever comes, with remembrance and awareness. Remembrance here means remembering the object of meditation. Awareness means being aware of your mind, whether it is distracted or not.
You have to study the teachings on shi-nä well. The teachings in Liberation in the Palm of Your Hand are regarded as very helpful.
With much love and prayers...