http://nickveenhof.be/blog/simple-guide-install-apache-solr-3x-drupal-7
Download the Apache Solr 7.x-1.x-dev module from the Apachesolr module and place it in your drupal folder for contributed modules. Typically this is sites/all/contrib
Enable ApacheSolr and ApacheSolr Search modules in admin/build/modules
Download Apache Solr from a mirror near you : http://www.apache.org/dyn/closer.cgi/lucene/solr/3.5.0
Unpack it somewhere outside your drupal installation and outside your web root/folder. Suggestion would be ~/Servers/apache-solr-3.5
Go to sites/all/contrib/apachesolr/solr-conf and copy all files to ~/Servers/apache-solr-3.5/example/solr/conf
In ~/Servers/apache-solr-3.5/example/solr/conf you can remove schema.xml and rename schema-solr3x.xml to schema.xml. Do the same for solrconfig.xml
Open your command prompt and do cd ~/Servers/apache-solr-3.5/example
Type the following command to start the Apache Solr service
java -jar start.jar
If all went well your Solr Server is running now! Congratulations!
Test your solr server admin interface by visiting http://localhost:8983/solr/admin
Go to admin/config/search/apachesolr/settings and click on edit. Verify if the url http://localhost:8983/solr/ is correctly entered and click ok
If you want Facets you should download Facet API and enable it. When this is done you can go to admin/config/search/apachesolr/facets and enable the facets you'd want
Careful because Solr comes unprotected. If you'd like to run Solr in a more permanent basis I would recommend to run it on Tomcat as described in my other guide http://www.nickveenhof.be/blog/setting-drupal-6-apache-solr-tomcat-6-and-ubuntu-910-karmic-koala and put an IP-filter in front of it.
if you want to run multicore you can type
java -Dsolr.solr.home=multicore -jar start.jar
Attention: All your files should be in multicore/core0/conf