An amaryllis bulb needs to be 1/2 to 2/3 out of the soil for the bulb to bloom. Placing it any deeper will not allow the bulb tip to get the light it needs to grow.
The easiest way to plant the bulb is to place soil into the pot until mostly filled. Place the bulb in the middle of the pot and push it into the soil until less than half of the bulb is covered in soil. Be sure the bulb tip is protruding above the edge of the pot. Add additional soil to the pot and press the soil gently around the bulb for best contact.
Once planted, the amaryllis will bloom in about four to six weeks. Amaryllis flowers prefer indirect sunlight while in bloom. Keep the soil moist but not wet so that the bulb does not mold or decay.