Assuming you've unzipped a fresh new release there are a couple of steps to take:
Setup a Solr core
On the Solr server make a core available for ArchivesSpace. Copy the solr files from the ArchivesSpace source into the core's conf directory and enable it:
https://github.com/archivesspace/archivesspace/tree/master/solr
Disable the embedded server Solr instance
Edit the ArchivesSpace config.rb file:
AppConfig[:enable_solr] = false
Note that doing this means that you will have to backup Solr manually.
Set the Solr url
This config setting should point to your Solr instance:
AppConfig[:solr_url] = "http://solr.somewhere.org:8983"
Include path if required:
AppConfig[:solr_url] = "http://solr.somewhere.org:8983/solr/archivesspace"
You should monitor the ArchivesSpace logs and Solr to ensure that the indexer application is connecting to the external Solr instance.