Measure the width length and depth of each existing stair tread. No two stairs are perfectly alike, especially if the house is old. Order pine stair treads at your local home improvement center. They will offer you a standard size, but you may have them custom-cut to your measurements. Number each tread on the bottom as they are cut.
Stain the pine treads to match the risers and other hardwoods in your home. Wipe on stain on with a paintbrush and wipe off excess with a soft cloth, brushing with the pine's natural grain. Let it dry according to label specifications. Scuff the surface with fine-grit sandpaper between coats, applying as many as you need to achieve the desired color. Finish with several coats of water-based polyurethane to protect against stains and scratches.
Starting with the bottom stair, pry up the old stair treads, one at a time, with a prybar. Many treads are also glued, so this will be a challenge. Use a hammer and chisel to knock them loose if necessary.
Spread a bead of construction glue along the top edge of the riser, the sides and the back of the stair opening. Construction glue comes in tubes designed for this purpose. Fit the pine tread over the old stair and lean on it hard for 30 seconds. Hammer in finishing nails, 3/4-inch from each edge, at 2-inch intervals.
Move on to the next stair, repeating Steps 3 and 4 until you reach the top.