To centre text, enclose the material to be centered between
\begin{center}
and \end{center}
. For example:
centred text
achieved by:
\begin{center} centred text \end{center}
Note that, in figure environments, you should use a different command
to achieve a centred figure, namely \centering
(with no begin and
end expressions). Refer to the source for figure 3.1, for example,
to see how this is done.