How about some more detail?

What are you wiring?
What is the draw?
Why #8?

What exactly do you mean by "how"?

Stan,
Thank you for takeing the time to answer my question.
I am wiring a cooktop that is either 208 or 240 volts. Electrcian ran wire but is gone long story. Am homeowner with my carpenter who has done wiring in the past. Am confused because always thought you needed three wire for 220. Was told you can use white wire as second leg rather than as neutral. Is that corect?

If the cooktop is straight 240V you don't have a neutral, black and white are conductors. Retask the white through the breaker and mark it with black tape. If the cooktop is 120/240 you will need a 3 conductor with a neutral.