Double or one-half spacing

Question

How do I double space just a portion of a document, and not the entire thing?

How do I double space an entire document? What package should I use?

Answer 1

How do I double space an entire document? What package should I use?

As commented, one option is to use the package setspace:

\documentclass{article}
\usepackage{setspace} 
\doublespacing
\begin{document}
One line\par 
Another line
\end{document}

But note that \doublespacing is not equivalent to the double spacing in Word. I explained some alternatives to obtain that in Number of lines in double spacing (compared to Word).

How do I double space just a portion of a document, and not the entire thing?

The easier option to limit the double spacing effect is also the package setspace:

\documentclass{article}
\usepackage{setspace}
\begin{document}
One line\par 
Another line\par
\begin{doublespace}
One line\par 
Another line
\end{doublespace}
One line\par 
Another line
\end{document}

If you need a Word-like double spacing for some paragraphs use:

\begin{spacing}{2}
.......
\end{spacing}

Another option without packages could be change the \baselineskip inside a minipage:

\begin{minipage}{\textwidth}
    \baselineskip=2\baselineskip
     .....
\end{minipage}

Or simply change the \baselineskip in the main text two times:

\baselineskip=2\baselineskip % start double spacing
.......
\baselineskip=.5\baselineskip % start single  spacing