It's simple. As soon as ARDOR launches there will be a snapshot for all NXT accounts. Then, you download the Ardor Wallet and just log in with your current NXT passphrase in your new Ardor account. There you will see your Ardor and your IGNIS.
Example:
Your NXT account ist XXX-XXX-XXX-XXX-XXX and your passphrase is "Openthedoor". Let's say you own 1900 Ardor Assets and 1900 NXT in this account.
When Ardor launches and goes live, you just download the Ardor wallet and use your NXT Password "Openthedoor" to log in your new ardor account. It will have the same name as your NXT Account XXX-XXX-XXX-XXX-XXX. You don't have to do anything.
In your new Ardor Account you will find 1900 Ardor and 950 IGNIS (0.5 IGNIS for every NXT you hold).
After that, the old Ardor Asset in your NXT Account will be deleted (as far as I know). You keep your NXT and can hold it or sell it. Up to you.
I don't know if you can hold your NXT/Ardor on exchanges during the snapshot. There is no info about this yet. But the developer team will for sure give some info about this before the snapshot. It was no problem to hold NXT on Polo for example during the Ardor Snapshot last year. Maybe they will do the same with IGNIS. But it is safer to just hold it in your NXT Wallet.
I hope this answered your question. If not, just ask me and I will try to help :)