Modern english-speakers don't use "nay" anymore. Instead, we simply use "no." But, below is how you can use it.
Did you murder the king?
Nay, I would never even entertain the thought!
How long will it take for her to regain her ability to walk?
It will take months. Nay, years.
Who are the naysayers who opposed my ruling?
<naysayer means a person who disagrees with something, opposes something, or simply says no>