Fixed layout design always contains items in boxes/tables/DIV's whose measurements are in pixels or 'absolute' values.
To make a 'liquid' layout, use percentages.
i.e. a centred table with a width of 700px will ALWAYS be 700px sitting in the middle of the screen. A centred table with a width of 80% will always fill 80% of the screen.
(I use tables as an example. Most newer sites use CSS Divs, same principle)
Inside a table, elements such as sidebars can be constrained to a particular size, wheras the body cell can be the flexible one.
A simple source for a FIXED table would look like:
<table width='700px'>
<tr>
<td width='180px'>This is the sidebar</td>
<td width='520px'>This is the body text content</td>
</tr>
</table>
However, a LIQUID layout table could look like:
<table width='90%'>
<td width='180px'>This is the sidebar. However wide the main window, it will always be 180px wide</td>
<td>This is the content - we haven't specified a TD width, so it will always fill the remaining space within the table</td>
</tr>
</table>
It's important to understand div or table constructs and absolute (i.e. fixed) measurements and variable (i.e. flexible) measurements.
As an example...
I've done a sample set of tables at:
http://www.nothingupmysleeve.co.uk/layouts.html
Experiment with making the window narrower and wider to see the effects, I've done 3 tables. Take a look at the source code to see how it's achieved.