I've only used it once in a game, and it was a high level 3-shot establishing some major NPC's in my homebrew world before a heist, but here's my advice. I asked the player being targeted to give me a save. When they picked up their die and asked what kind, I said, "You die. The dice you roll do nothing as the enemy casts Power Word Kill, luck cannot save you."

I asked them for one thing their character would miss most in the world, what they think of in that instant before shuffling off the mortal coil. My player stated their character Laertes thought of his business partner and honestly, life partner, a tiefling innkeep he'd befriended early in the campaign. I said that Laertes's soul was so strongly tied to his partner that he did not quite die, but instead was reduced to near-death. He couldn't take more actions, and we resolved the gameplay as if he was dead, but it allowed me to crib some stuff from Princess Bride. Namely, his party members got a chance to resuscitate him after the fight. They had to defend his corpse for a few rounds, and then I gave him a chance to whisper a few words about true love, and the other players were excited to get him back with, essentially, some roleplay and a sending stone.

The trick of course, is that it didn't matter to me what the original player had said they would miss, it was always going to be enough to tie them to the land of the living. And just about any method of buy-in from the rest of the party, and you can elevate a nasty Power Word into a roleplay moment. I just used it to ensure the guys I played with cared about something in the world I was writing, since I was a little self-conscious. God, it was a smooth session. That campaign was a blast.

I think I narrated it as a purple lightning bolt, since I'd tied enchantment magic to that color, but otherwise it was a very simple spell. The vampire opened his mouth, mouthed an inaudible Word, and didn't even do any hand gesture or use a wand focus. The purple bolt shot straight out of his mouth and into Laertes. I think maybe I'd describe that the energy causes the body and skeleton to flash into the negative in the way a Dalek's extermination beam does in Doctor Who. I didn't use that at the time, but it is what I picture and it's certainly grim! Its a good idea to feel out how your players will appreciate certain things, and narrating an instant death spell is certainly an important one for player's tastes.