This PDF brings back memories. I went down this path about a little over a year ago—it’s great fun. I didn’t end up finishing anything significant but I’d highly recommend everyone give writing a kernel or an OS a shot sometime. You will learn a whole lot.
If you’re interested, the OSDev Wiki [0] is a great place to start, as are Bran’s Kernel Development tutorials, which I can’t seem to find on mobile right now.