When working under the eyes, have your model look upwards - only with the eyes, without moving the head.
If you work above the eyes (eyelid), have your model close his/her eyes in a relaxed manner.
Children are inclined to shut their eyes tight. Then you can say: keep your eyes closed as if you are sleeping.
To make the eyes appear slanted, you proceed as follows: Draw with a fine brush to draw a black line along the upper edge of the eyelashes. Then extend that line diagonally upwards from the outer corner of the eye. Then extend it the other way, from the inner corner of the eye downwards.