Scoreboard confusion

ive been trying to get a mob in my mod to do something after killing a certain amount of mobs, i have tried putting it into the mobs "if entity kills another one" trigger to put a point onto a scoreboard, so that i can use that to count how many it has killed and then do a procedure once the needed threshold is reached. yet it doesnt seem to be wanting to work. is it that i need to make some sort of tag for the "kill_count" scoreboard to make it work and start counting? or is it something else ive done wrong like setting it up in a strange way?, id appreciate if someone could tell me how exactly setting up a simple scoreboard like this could be done and how the scoreboard can be used to trigger an action once it has reached its needed number. no matter who asks about scoreboards from what ive seen never get an answer from anybody about scoreboard related things as if its some evil knowledge lmao, would love to even get a link to some sort of scoreboard tutorial, personally have not found a single one, thanks!