You need to know where the apxs tool is. apxs is part of the apache2 webserver and needed to compile mod_tidy.
$ tar xfz mod_tidy.tar.gz
$ cd mod_tidy
$ ./configure --with-apxs=/path/to/apxs
$ make
$ su root
# make install
That compiles mod_tidy and installs the module into the httpd module directory. Now you need to configure the webserver.
Add the following directives to your httpd.conf file or into a <VirtualHost> section.
The path to mod_tidy.so
depends on your apache installation.
LoadModule tidy_module /usr/local/httpd/modules/mod_tidy.so
AddOutputFilterByType TIDY text/html application/xhtml+xml
So the webserver loads mod_tidy and uses mod_tidy to filter HTML and XHTML output.
To set TidyLib options you use the TidyOption directive. An example mod_tidy configuration:
<IfModule tidy_module>
# Run all HTML and XHTML through mod_tidy
AddOutputFilterByType TIDY text/html application/xhtml+xml
# TidyLib Options
TidyOption accessibility-check 3
TidyOption char-encoding utf8
TidyOption enclose-text yes
TidyOption indent auto
TidyOption logical-emphasis yes
TidyOption output-xhtml yes
TidyOption wrap 0
</IfModule>
For other TidyLib options refer to the TidyLib documentation