Instructions
Gear your Shadow Priest efficiently. Look for gear that has increased spell damage, intellect, and spirit stats.
Work on increasing your stamina so that you can take more hits without dying as you are casting.
Spend talent points in the Shadow talent tree to increase damage inflicted by shadow spells and to decrease casting times when using shadow spells, for example.
Spend talent points on Shadow form when you spend your 31st talent point.
Download a threat meter program to monitor your threat level in instances and raids. Some examples are Omen and KTM. Keeping your threat level low will help you take less damage.
Stick to a spell rotation in group situations to inflict the most damage per second (DPS) while still providing the best help possible to the other members in the group. One example of a spell rotation is: Vampiric Touch, Shadow Word: Pain, Shadowfiend, Shadow Word: Death, Mind Blast and Mind Flay, although Devouring Plague, Dark Archangel, Mind Spike and Mind Sear can all be incorporated into this rotation successfully as well.
Always aim to provide the maximum amount of utility to your group rather than concentrating on damage.