How do I activate the Genocide Route in Undertale Ruins

I am playing Undertale at the moment, and so far I have done Pacifist and Neutral a couple of times, but now I want to do genocide. But, for some reason, I cannot activate the genocide route. Why is this? I cannot get the 'But nobody came' thing. Is it because I didn't do a true reset? I just reset. I killed Flowey in a neutral 'no mercy' run. Please answer! Also, does it matter that I only reset, I didn't do a true reset? Thanks! By the way, where is the best spot to find monsters to kill?

You can start a genocide run whatever you did in your previous runs. To activate a genocide run, you first have to kill all the enemies present in the Ruins (a total of 20 enemies). When you will do so, the message "But nobody came" will appear during your next battle, as you said. The music will also change and will confirm that you are ready to go kill Toriel, and thus start your genocide run.

What usually happens when you are trying to start a genocide run in the Ruins is that you end up not finding any monster anymore because they become really rare. You have to keep walking until you kill all of them though. It can take a bit of time, so don't worry if you walk for several minutes without meeting any monster.

Do note that you are allowed to spare some monsters, but you still have to kill the amount required (20 in the Ruins as said) to unlock the genocide run.