Download the new wallet/daemon from: http://seeds.dero.io/testnet/ - choose the windows_amd64 binary - and unzip to a new folder.
Next create a new subfolder called "testnet" in the above folder.
Download the database.db snapshot from http://seeds.dero.io/testnet/blockchain/derod_database.db.zip - and unzip it to the testnet folder.
Right click on the derod executable and set to "Run as Administrator" in the compatibility options.
Run the command:
derod-windows-amd64.exe --testnet
to begin syncing with the testnet.
While the daemon is syncing you next need to generate a NEW wallet address specifically for the testnet - any coins mined on the testnet will not be valid once the mainnet goes live (ie - they aren't worth a damn thing). Later on when the official Atlantis wallet comes out [I think!] you'll want to use the "restore from seed" option to import the balance from your existing DERO wallet address to the new Atlantis wallet.
Run the command:
dero-wallet-cli-windows-amd64.exe --testnet
Then select option 2 from the menu - create new wallet
Enter a name for the testnet wallet
Enter a password for the wallet twice
Save the recovery seed
Display the wallet address - save address
Once the daemon has finished syncing enter the following command string at the daemon prompt to begin mining on the testnet:
start_mining
Replace
Et voilla, you are now mining completely worthless coins for the good of the DERO network!
EDIT - forgot to mention that Atlantis is exceptionally sensitive to time-sync errors; the easiest way to fix that should the daemon complain your clock is out of whack is to open up the time and date settings, uncheck "set time automatically" then re-check it. That will force an update with the M$ timeserver. Or you can be a masochist and attempt to change the timeserver to pool.ntp.org; good luck with that.