:~# apt-get install apache2 libapache2-mod-php5 php5-sqlite sqlite3 php5-gd
:~# a2enmod php5
:~# a2enmod headers
:~# a2enmod rewrite

:~# mkdir /var/www/selfoss
:~# cd /var/www/selfoss
:~# wget http://selfoss.aditu.de/selfoss-2.6.zip
:~# unzip selfoss-2.6.zip
:~# chown -R www-data:www-data /var/www/selfoss
:~# chmod g+w data/cache data/favicons data/logs data/thumbnails data/sqlite public

:~# vim /etc/apache2/conf.d/selfoss.conf
And put there this (see http://selfoss.aditu.de/forum/index.php?id=7 for details):
<Directory /var/www/selfoss>
AllowOverride All
</Directory>

Uncomment RewriteBase /selfoss in /var/www/selfoss/.htacces

:~# service apache2 restart