Here's what we started with:
See the cracking, peeling, and general dirtiness? Also, see that adorable photo bomber?
First I used a scrapper to remove as much chipping paint as I could.
Then, I sanded everything I could get to with the electric sander. Mostly the tops of all the rails. I didn't worry about sanding in between all the rails. The paint wasn't chipping in between and I wasn't switching colors or anything.
Then, I made some bleach water and cleaned everything- every nook and cranny. That took a while. It was already looking better. I didn't get a picture of that.
Here's a mid-progress picture. The right was done and the left wasn't. See the difference?
I also painted the trim around the door because it was looking really rough. This is an older before picture I had:
And after (the horseshoe's been gone for a while)
It's so much nicer walking up our stairs now!