Looking for A guide for a Friend

[Sephiroph]

A friend of mine recently joined my mythic raiding guild as a shadow priest and is really struggling with keeping up in damage. I want to help him, but I'm a DK and don't know jack about Shadow Priests. Could anyone point me towards what is considered to be the best DPS guide? I've seen guides on icy-veins and wowhead but I don't have the background to know if they are up to date/worthwhile. Thanks for helping me help a friend.

[fhymn]

howtopriest.com
I think they have a discord channel aswell, which should be more up to date. If not, ask somebody there.