Here are the steps to follow to apply varnish to a wooden floor:

1. Prepare the wood: Before starting, make sure to prepare the wood by sanding it with the recommended grain from the varnish manufacturer. If there are any stains on the surface to be varnished, make sure to remove them before starting.
2. Stain the wood (optional): If you wish to stain the wood, choose a stain and apply it following the instructions on the label. Wait for the stain to completely dry before continuing.
3. First coat of varnish or sealer: Apply a first coat of varnish or sealer following the manufacturer's requirements to seal the wood. Wait for this coat to completely dry.
4. Light sanding: Once the first coat is dry, lightly sand with sandpaper to smooth the surface. The recommended grain varies depending on the product but is usually P220.
5. Apply a second coat of varnish: Dust the surface and apply a second coat of varnish. Wait for the second coat to be hard enough to walk on before applying a third coat, without needing to sand between the two coats if you follow the recommended application time, which can vary from 2 to 3 hours.
6. Apply a third coat of varnish (optional): If you wish, you can apply a third coat of varnish as soon as the second one is hard enough to walk on, without needing to sand between the two coats.

By following these steps, you can achieve a professionally varnished wooden floor.