Currently I need to implement custom solr similarity. So I found out that I need to override the DefaultSimilarity class in order to do this. Still I can't figure out how exactly it should be done and where to get source code which can be used for this purpose. Any help would be appreciated!
For anyone who will need an answer:
What I needed to do was to create a package project in eclipse, download lucene-core jar and add it to the project. After that I imported the needed library and overrided the Similarity class.
After that I created a jar with the new similarity class. Added the jar to the solr/dist/ directory. Added a ref to the jar to the solrconfig file for the needed core. Used the similarity like any other similarity in the shema file.