Make a template for the carpet stair tread based on the step size. Measure the area of the step you want covered with the tread and transfer those measurements onto cardboard. Cut out the template and lay it on the step to get an idea of the appearance. Trim away any excess for adjustment if necessary.
Lay the carpet remnant flat on a work bench with the fibers facing downward. Put the template on the carpet back and trace around it with a marker. Repeat the tracing process for each stair tread you’re making.
Cut each stair tread out by pressing downward along the marker lines, slicing with a carpet razor.
Attach the carpet remnant stair treads to each step. Apply double-sided carpet tape onto the steps and press the tread on top for easy future removal. Apply a layer of carpet adhesive to the bottom side of the tread and press it into place when you seek a more permanent attachment.