Carpentry and Woodworking - New bottom step

I had a fire in a house of mine and I need to replace the bottom step. Should I just go to Home Depot and buy the Oak tred, riser etc........Is there a trick to making the bottom riser. I looked at the orig and it was cut on the back every1/4 inch. Do I need to soak it also ???? Oh ya the bottom step is rounded at the ends
A pic is attached. I was looking for a house that was being torn down but no luck
http://www.myfishpix.com/gallery/showphoto.php?photo=5827&cat=500&page=1

One way to do this is to build a frame out of dimension lumber to the correct height less the thickness of the tread material. Round off the framing on both ends. You have one end to use as a template, so use it. After the frame is in place and secured to the subflooring and to the riser behind it, you will need to install the riser. Now, it would be better for you to use 3/4" oak veneer plywood, sometimes called Sandeply. Cut it the proper length and height including the rounding part. Place it upside down and cut kerfs about every 1/4 or 3/8 inch apart the about 1/3 the width of the plywood on each end about 1/2" deep. This will allow for the last two plys to be bent in the configuration you need. It would help if you could apply steam to the laminations, thereby softening it for a smooth curve. The last ply will tend to crack if you don't. Once it is pliable, set in in front of your framing and nail it in place starting from the center and moving to the ends. All that will be left is to install the proper tread. The width will be determined by the width of the frame, riser plus about 1" bullnose. Post back if we can help further.

Thank you- makes perfect sense. The only question I have is about the tread. I looked at them at the Depot. They are oak and rounded along the front. Do I need the bullnose? The tread was also rounded on the sides.
Do I use a router to match what is allready cut? Did I confuse you?
Thanx

No, you need to match what you had originally, and routing the front and side makes good sense. Good luck with the restoration.