What's the best way to transition to friendship after going on a date with someone?
Well… I only recommend this move if you honest-to-God, really want to be friends with them. If you just don’t want to date them, instead of converting to “friends” I recommend you say, “We’re not quite a match for dating” and move on.
But if you totally enjoy them as a human, and you want to hang out with them, just not romantically, then tell them you want to be friends, and ask them out on a friend date in the same breath. It could look like this: “Hey, I don’t think we’re a match romantically, but I want to be friends — seriously. Do you want to go see the Roots concert with me at the Fox on August 14th? If you don’t want to hang out as friends, I understand that too.”
Don’t assume they want another friend; that’s most likely not why they dated you in the first place.