Rule 1
Who refers to people. That and which refer to groups or things.
Rule 2
That introduces essential clauses while which introduces nonessential clauses.
Rule 3
If this, that, these, or those has already introduced an essential clause, use which to introduce the next clause, whether it is essential or nonessential.
Rule 1
First of all, use the ever suffix when who or whom can fit into two clauses in the sentence.
Rule 2
Because we can substitute him and he in both clauses, we must use the ever suffix. Now, to determine whether to use whoever or whomever, here is the rule:
him […]
Rule
Use the he/him method to decide which word is correct.
he = who
him = whom