First choose the Relic set that has the best set bonus for your Primal. Each Relic set will give a passive bonus when you have 2 and 4 of that set equipped. After that you will need to select the Relics with the best starting main stat and substats that fit your Primal. Level up those Relics to see what substats get upgraded. Always be on the look out for Relics with a better set of stats.