Hey there. So the idea is you don't actually roll on them -- you just use the number that took you down below 0 hp instead. So say you took 3 damage from a sword and that took you below 0: just use entry 3 on the slashing table.
I've also seen people using the negative hp value (if you're down to -2, use entry 2), or rolling the same dice you used for damage, tho. It all works out fine.