Eplain defines three commands to conveniently justify multiple
lines of text: \flushright, \flushleft, and
\center.
They all work in the same way; let's take \center as the
example. To start centering lines, you say \center inside a
group; to stop, you end the group. Between the two commands, each
end-of-line in the input file also starts a new line in the output file.
The entire block of text is broken into paragraphs at blank lines,
so all the TeX paragraph-shaping parameters apply in the usual way.
This is convenient, but it implies something else that isn't so
convenient: changes to any linespacing parameters, such as
\baselineskip, will have no effect on the paragraph in
which they are changed. TeX does not handle linespacing changes
within a paragraph (because it doesn't know where the line breaks are
until the end of the paragraph).
The space between paragraphs is by default one blank line's worth.
You can adjust this space by assigning to \blanklineskipamount;
this (vertical) glue is inserted after each blank line.
Here is an example:
{\center First line.
Second line, with a blank line before.
}
This produces:
First line. Second line, with a blank line before.
You may wish to use the justification macros inside of your own macros. Just be sure to put them in a group. For example, here is how a title macro might be defined:
\def\title{\begingroup\titlefont\center}
\def\endtitle{\endgroup}