The fastest way to travel from Paris (Gare du Nord) to London St. Pancras Station is by taking the Eurostar train. Eurostar is a high-speed train service that operates between major cities in Europe, including Paris and London. Here’s what you need to do:

1. Start at Gare du Nord in Paris: Make your way to Gare du Nord, which is one of the main train stations in Paris. It is easily accessible by public transportation, including the Paris Metro.
2. Eurostar ticket: Purchase a Eurostar ticket for the journey from Paris to London. It is recommended to book your ticket in advance, as availability can vary.
3. Security check and immigration: Before boarding the Eurostar, you will go through a security check and passport control. Make sure to arrive at the station with enough time to complete these procedures.
4. Board the Eurostar: Once the security and immigration procedures are done, proceed to the designated platform for the Eurostar. Listen for announcements or check the departure boards for the correct train.
5. Travel through the Channel Tunnel: The Eurostar travels through the Channel Tunnel, an underwater tunnel that connects France and the United Kingdom. The journey takes approximately 2 hours and 20 minutes.
6. Arrive at London St. Pancras Station: After the train ride, you will arrive at London St. Pancras Station, which is centrally located in London. From there, you can easily access other parts of the city using public transportation or taxis.

It’s important to note that travel times may vary, so it’s advisable to check the Eurostar website or contact their customer service for the most up-to-date information regarding schedules and journey duration.