This is a detailed guide on how to setup and backup your qt-wallet properly. I recommend to perform all these steps before you send any funds to your wallet.

I wrote this tutorial with the DeepOnion wallet but it should work with every other qt wallet (like bitcoin-qt, litecoin-qt, etc). So, you can find this tutorial also on the DeepOnion forum
https://deeponion.org/community/threads/tutorial-how-to-backup-your-entire-wallet-and-all-private-keys-including-change-addresses.23622/

Setup

This is straightforward. Go to the DeepOnion Website and download the newest version https://deeponion.org/#download (DeepOnion, anonymous cryptocurrency, anonymous transactions).

Open the wallet and wait for the sync to finish. Then encrypt the wallet under Settings -> 'Encrypt wallet'. You will get a security warning and then you have to reopen the wallet. Test to unlock your wallet, before you proceed.

Backup Wallet

Before you begin: Open your wallet, go to 'Receive Coins' and create some new addresses for future use.

Under File select 'Backup Wallet' and save the encrypted wallet.dat to a secure location. You will need your password to open the file.

You don't want to only backup your wallet.dat, because in rare cases the file can become corrupt. Then you probably have no chance to access your funds again.

Backup Private Keys (Do not use dumpprivkey)

You should not use dumpprivkey to extract your private key, because you will only get the private key of your main address. If you create a new address or a change address is automatically used, you will lose your funds.

Instead you can use dumpwallet

Open your console under 'Help' -> 'Debug Window'

Make sure to unlock your wallet first and then you can simply type 'dumpwallet deeponion_privatekeys.txt' in your console.

After you hit enter you will get this useful file. It will be saved in the same folder as DeepOnion-qt.exe and contains all private keys, including your change addresses and reserved addresses. So you can create a limited amount of new addresses and still have the private keys.

Where should I store my backups?

This is an important one. I would use a password safe like https://keepass.info (KeePass Password Safe) to save backups. Use one without a sync feature, you don't want your private keys in the cloud. Make some backups and make sure you don't forget / lose your password and masterkey file (I would not use the 'windows account' feature to unlock the wallet).

If there is demand for a detailed keepass guide, please let me know.