stair tread molding/nosing options
That bit of decorative bullnose on the end is called a 'return'.  If you haven't cut up all the treads you can return (no pun intended) some of them and buy treads with them already present.  They cost about 4× as much, but you can buy them.

Cheapest thing to do would be to use a router with a bullnose bit on the overhanging edge to create the return.

You can go for using the bullnose from the waste.  You've already got the material, and it's certainly going to match what you already have.  it will require you to make a small miter cut on the outside edge of the stair, with another crosscut from the back to the endpoint of that miter cut.

Personally?  If you haven't got the router, I'd go for buying the return molding on its own.  It's only about $5 per 13" piece at the Big Orange Box (i.e. Home Depot) or eBay