How to use guides?

Question

Design

Here's how I can not understand! Here's how to set guides for website design? (Well, I know how to set guides, but for example, a site bar, here's how to calculate how much in width and height it should stand, etc.!)

Answer 1

Guides are made for front-end developers. Although they also help the designer to line up the elements of one "caste" in a common line.
In general, I try to throw almost all elements of the layout under the guides. In any case, this will simplify the life of the layout designer.
As for the size of the sidebar and other elements, you can use the regular grid here. For example 960.gs

Answer 2

Well, 960 only helps in width and that's it!

Answer 3

What is the question? There are no rules a la "top menu must be exactly 50 pixels high". Do as you like, and use guides so that all elements are aligned, and the indents between them are the same.