Assuming you need to replace a rotted subfloor in your kitchen:
First, identify the source of the water damage and repair it. Then, remove all of the flooring material from the area, including any baseboard or trim. Next, use a circular saw to cut out the damaged section of subflooring. Be sure to cut along joists so that you have solid wood to nail or screw your new piece of subflooring into.
Now it’s time to install your new piece of subflooring. If you’re using plywood, be sure to use marine-grade plywood, which is treated to resist moisture. Secure the new piece by nailing or screwing it into place along the joists. Once that’s done, apply a bead of construction adhesive around the perimeter of the patch, then lay down your flooring material and secure it according to manufacturer’s instructions.