The opt-in process is fairly straightforward. See this for detailed information:
https://nemplatform.com/symbol-migrationDownload the wallet with the opt-in module:
https://nemplatform.com/wallets/#desktopInstall on your machine. Open the wallet, login.
Go to services, go to the opt-in option and follow the opt-in wizard step by step.
The opt-in process will generate your XYM wallet address, your mnemonic passphrase, and VFR keys (if you will be using for harvesting).
Save these to a safe backup location, opt-in complete.
You need 100 XEM minimum in your wallet, there is a transaction fee of around 1.05 XEM.
Keep your XEM in your XEM wallet until the time of snapshot to receive free XYM at Symbol launch.
Any transactions after the snapshot will not count towards free XYM.