I made it the same way as I made the Dumbledore robe, but without the trim. I planned to put a Hogwarts crest on (by finding an image online and using iron-on printer paper), but I realized that I forgot to do so before taking the pictures. You can imagine that it's there in the picture, right? :)
The tie is a cheap (ugly knit) tie I found at Goodwill. I added stripes with some puff paint. Easy peasy. The rest of the outfit is just his Sunday clothes that he already owned. And, of course, we drew a little lightning-bolt scar on his forehead with some eyeliner. And, he doesn't need glasses, since he already wears glasses! This is possibly the easiest costume I've ever put together for my kids. Yay for easy!