I recently setup a Drupal 6.x installation with the apachesolr module. It was not as straightforward as I was expecting so I've made a few notes here. The most frustrating part is the Tomcat & Solr setup, after that the Drupal module works flawlessly.

I used this wiki page about Solr and Tomcat as a basic guide. First thing to do is to download and setup tomcat. I used the setup script we use at work for Tomcat (available here) to download and install Tomcat 5.5.27 in /usr/local/tomcat. Note that you can use any servelet container like Jetty, JBoss etc.