The Destiny 2 Steam version is now available, which means the Battle.net version is done and dusted. So if you want to continue playing on PC, you'll need to bring your account to the game's new home. Fortunately, setting up your Destiny 2 Steam account is pretty easy.
Firstly, head to Bungie's official Destiny 2 Steam page and click "Get Started". You'll need to sign in if you haven't already. From there, confirm your email and your account, then accept the transfer agreement. There are a few important things to note here. Firstly, this transfer is one-time and non-reversible - obviously, seeing as how Destiny 2 is leaving Battle.net after Bungie's split with Activision, AKA Battle.net owner Activision Blizzard. Secondly, the account linked at the time of the transfer is the one that will be moved, so if you happen to use multiple Battle.net or Steam accounts for whatever reason, make sure they're in order before October 1.
Once you accept the transfer agreement, you'll be prompted to link your Steam account. You'll just need to sign into Steam for this step, but again, make sure it's the account you plan to use to play Destiny 2. From there, you just need to review and finish the setup. Before you finalize it, make sure all of your Guardians and DLCs are showing up correctly, as some players have reported missing licenses. These issues seem to have been resolved after some day-one troubles, but it's worth checking.
If everything's in order, finalize the account transfer. Voila! You're good to go. Log into Steam, redownload Destiny 2, and you should be able to log into your account like normal. Note that you will need to re-add your Battle.net friends on Steam, which is a bit of a drag, but what can you do? At least we can join fireteams through chat commands now (try /id and /join with the number you get). Give and take, as they say.
Here's everything you need to know about setting up Destiny 2 cross save.