ok (a receiving one, I assume you mean?)
Yes, so you can send the coins to your Nano.
First, do you mean install a BCH wallet on my PC? Like Electron Cash?
Yes, install Electron Cash or any other trusted BCH wallet on your PC.
Second, what is mnemonic code?
Mnemonic code = backup seed. That list of words you get when you are creating your wallet. If you don't have it, just open your Electrum wallet, go to Wallet -> Seed, copy it and restore it on the Electron Cash wallet.
Third, you mean import it from Electrum (which should now have a balance of zero, having previously sent my BTC to my Nano)?
Yes. It has a balance of 0 BTC, since you took it out of there, but it still has the BCH. When you restore it on the Electron Cash wallet, you will be able to see the BCH balance and spend them to your Nano receiving address.