The following are the steps to use the polyurethane curing agent:

Prepare your work area. Make sure you work in a well-ventilated area. Wear gloves and goggles to protect yourself from chemicals.
Mix polyurethane and curing agent. Follow the directions on the product label to determine the correct ratio of polyurethane to hardener. Mix these two ingredients thoroughly until they are fully combined.
Apply the mixture to the surface to be painted. Spread the mixture evenly over the surface using a brush, roller or sprayer.
Let the mixture cure. Cure time will vary depending on the type of polyurethane you are using. Follow product label directions to determine cure time.
Once the mixture has cured, you can sand it, prime it and paint it if desired.
Here are some safety precautions you should take when using polyurethane curing agents:

Avoid contact with skin and eyes. In case of contact, rinse with water immediately.
Do not breathe vapor. Use in a well-ventilated area.
Keep this product out of reach of children.
Polyurethane curing agent is a dangerous substance and must be handled and stored properly. Follow product label directions for safe handling and storage.