Once you're back in the country on your spousal, you don't actually need to APPLY for a NH (I assume you mean NHS number or medical card).
You just go down to your local surgery and tell them you want to register. Seems like different surgeries have slightly different procedures, but you'll want to take your passport along. They should ask (but the two surgeries I've registered with didn't) about your status, and it's good to be prepared to let them know that you are entitled to register and use the service.
A few weeks after registering, you should receive a card in the post, which, in my experience, can then be safely tucked away again with your other important documents. The number on the card is registered at my doctor's surgery, and I've never been asked to produce the card...