THey are generally narrow crowded streets with shops, beggars, carriages, etc. Also, generally the weather is grey and dull and often raining.
That is some great advice!!
When I do something like this, I look at a picture (try googling "baker street" "Holmes" and look at the google images.) Then, take several deep breaths to oxygenate your brain, and wait for some words to come. Type a lot of words, whatever you think of, brainstorming.
Give the reader something to see -- a leaf falling past a storefront window, touching the ground briefly and then haphazardly leaping across the street. Let the reader see all that it passes, and when it lands somewhere, let the reader land there, too. Be the voice of the reader's mind as the reader stands there within the scene.