He spent months in his garage building a custom, life-size Tardis with the question "Will you marry me?" at the top panel above the door. The build was as true to the Tardis from the BBC series as possible, right down to agonizing over which shade of Blue was true Tardis Blue. He managed to keep the garage build a secret from his girlfriend by convincing her that he was instead building her an epic Halloween display, and not allowing her in the garage for two months.
When he proposed, he gave her a wedding ring and the key to the Tardis, though he sadly didn't find a way to make it bigger on the inside. Now, he said, she has to pick out the Halloween decorations.
Of course, he captured the entire proposal and her priceless reaction on video for the world to see. Check out the video, which will give you all the feels, at the top of this post.