Support for Cyrillic is based on these standard LATEX mechanisms: the fontenc and inputenc packages (and on babel). Thus the basic principles for its use are similar to those for other European languages: you simply add, to your document preamble, lines like the following.
\usepackage[T2A]{fontenc}
\usepackage[koi8-r]{inputenc}
Here you can put any desired input encoding instead of
koi8-r: for example, it would be cp866 if you are
using a MS-DOS text editor with this Cyrillic code page to prepare your
documents, or cp1251 if you are a MS Windows user with Cyrillic
support. A full list of the available Cyrillic encodings can be found in
Section 5 and in the file cyinpenc.dtx.
Documents are, naturally, not restricted to a single font encoding;
this is essential for multi-lingual journals or documents. Such
changes can be made by using the \fontencoding command as part of a
font-change. However, it is best to access these font encodings via a
higher-level interface.
Since such changes are often closely related to other language-dependent settings, it is often sensible to use the babel system, which provides further useful `localisation' and standardised multi-lingual interfaces (for further details, see Section 2.3). Then you can use lines like the following in your document:
\usepackage[koi8-r]{inputenc}
\usepackage[russian]{babel}
This will automatically choose the default font encoding for Russian,
which is T2A, if available. Documentation of the complete set of
font-encoding selection rules can be found in cyrillic.dtx which is
part of rusbabel.
These LATEX interfaces are very convenient because they make your documents completely portable, being based solely on standard TEX features. This will mean that your documents can be processed on any TEX system without any need for re-encoding to the `native' encoding used on each platform; this is because the encoding of the document is specified in the document itself.
Moreover, if necessary, more than one input encoding can be used
within a document; this could be useful if, for example, you need to
combine articles prepared by authors on different machines. Each part
of the document is then identified by a \inputencoding command,
which can therefore only be used between paragraphs.
Please note that you must always use the two standard LATEX
commands, \MakeUppercase and \MakeLowercase to produce uppercase
or lowercase text in your documents. This is because \uppercase and
\lowercase will not work at all for Cyrillic (note that these latter
two commands are not, and never have been, available for use directly
in LATEX documents).