Firstly in order for this function to work you need to have cookies enabled. The cookies stored by this page contain the two numbers you will input at the bottom of the page, their latitude and longditude equivalents and nothing more. They are stored on your computer and not sent anywhere else.

For this example we shall assume your are the Prime Minster and you live at 10 Downing Street in London.
Open either Street Map or Multimap (these links open in a new windows)
Then in the search box enter in your street name, in this case Downing Steeet. This will take you to a list of options for the various Downing Streets around the country. Since you live in London choose the SW1 Version.

Below the map is a box labeled map information. In this box there are a set of numbers (in the case of Downing Street):
X:530100m Y:179900m.

Take these numbers and enter them in the boxes below

In a blue box towards the bottom of the page is a box with some copyright information. At the bottom of this box there is a line that reads:
Location is at 530067E 179949N (N51:30:13 W0:07:39).

Take these numbers and enter them in the boxes below.