I never thought about all the stairs in my two story home being an issue for dogs when I started fostering . . . until my first foster would not even walk the two low, wide steps into my house.
My first Sheltie taught herself to go up and down the stairs by carefully watching my other dogs do it. She would literally sit on the landing and watch them walk the stairs.
Most of the others have needed help. To teach them to go upstairs, I sit with a treat in my hand about two steps above them. When they reach me, they get the treat, then I move up a couple more steps, and repeat this until we reach the top.
I think going downstairs is scarier for them. To teach them to go down, I will hold onto them around their chest, and walk them down while I walk the steps behind them, praising them with every step they take. Of course they get a high quality treat when we reach the bottom.
I know we look very klutzy going down the stairs this way, but I think that by holding on to the dog, it gives them a sense of security.
The basement stairs into my office have been harder for the dogs, as they are steeper, narrower, and darker. But once they figured out the open staircase to get upstairs, they eventually figure it out on their own.