The most direct path is east to the A201 south, across the Blackfriars Bridge to Southwark and then "The Cut" west to Waterloo Road.
It is 6 minutes faster, on average, to use the A4200 due south across the Waterloo Bridge.
If you are travelling between London main line stations, I assume that you won't be travelling by car. Therefore, walk about 100 yards to Euston Station, get on the Northern Line (underground/tube) going south and it will take you directly to Waterloo station. Alternatively, if you have a huge amount of luggage, take a cab.
Take the Northern Line going south to Kennington and change to the Northern Line going one stop north to Waterloo.
Take the Northern or Victoria line to Euston (one stop) and then the Northern line southbound to Waterloo.
Yes, you can use the northern line.
From entrance to entrance 25 minutes.
Yes - the Northern Line (Charing Cross Branch)
King's Cross St. Pancras tube station was created in 1968.
Taxi is quickest and most expensive. Tube is cheapest but the journey is more complicated (you have to change lines).
The Northern Line (Black one)
The Hammersmith and City lines did not call at Kings Cross St. Pancras.
From Waterloo take the Bakerloo line to Oxford Circus. There transfer to the Victoria line to St. Pancras International rail station. Journey takes about 20 minutes with one change of train. Check the 'London Tube' site for service delays and restrictions.
Approximately 15-25 mins and £15
Look at your Tube Map and travel to Kings Cross/St Pancras Station.
Waterloo. There is an overground and underground station, both called Waterloo. There is also a station adjacent to the main station called Waterloo East. Waterloo International is now closed and its services transferred to St Pancras International.
Kings Cross/St Pancras