Fix deck stairs

We've got some stairs that need to be fixed at the front of our house. One of them is obviously broken and sagging, but the others need a bit of support as well. We had a handyman come and look at them a while ago and he pointed out that some of the nails are in the wrong spot, and he was suggesting a few places to add extra blocks and planks of wood. I can't remember all the details but I'm sure it will be pretty clear to you as well if you have some carpentry experience. Cheers