When you install have nothing else plugged in except the SSD, this is important.
Once installed you can wipe anything you don't mind losing, keep it for a couple of weeks perhaps to be sure, but then format the drive that had your old windows on, leave the games on the other drive, when you install steam tell it where your steam folder is, and it may check when you try and play, but that'll be it, ditto with other games providers.