Ask "How likely are you to recommend us?" on a 0 to 10 scale. Count promoters (9 or 10), passives (7 or 8) and detractors (0 to 6). NPS is the percentage of promoters minus the percentage of detractors, so it ranges from -100 to +100. For example, 50% promoters and 20% detractors gives an NPS of 30.
NPS = % promoters minus % detractors. Passives count towards the total number of responses but not towards either side, so more passives pull the score towards zero.
A positive score means more promoters than detractors. Benchmarks vary a lot between industries, so the most useful comparison is your own score over time, and the comments behind the numbers.
Yes. If you have more detractors than promoters the score is below zero.
It depends on the industry. Above zero is a start; track your own trend.
More is better. With only a few responses the score moves a lot from one answer.