To setup your Drupal instance with Opensolr, you can either use the Search API Opensolr module, or, you can use Search API Solr.

In both cases, the setup should be pretty straight forward.

In the first case, you also have the option for AutoConfiguration, where you won't have to worry about the Solr server setup, at all. 

All you'd have to do, is to create your index and start indexing your data.

The very important steps, however, if not using the Search API Opensolr module, are as follows:

1. Make sure you select the correct Solr Version, when creating your Opensolr index.
   Selecting the wrong version, that is not supported or otherwise doesn't work with your version of Drupal, may cause data loss, and render your index unusable.
   
   
2. Remember that all Opensolr Indexes are protected by Basic HTTP Auth.
   You will have to configure your Opensolr Index credentials, every time you update your Solr Server configuration.
   You can change your index credentials, at any time, from your Opensolr Index Control Panel.
   
   Remember that you should never use special characters in both your username and password. Those will be discarded and you may get the wrong credentials inside your Drupal Solr Server settings.
3. Pay very close attention, when downloading the Solr Configuration Zip Archive from Drupal, so that the downloaded archive will match the Solr version you are using inside Opensolr.
   Once you did that, you simply have to upload the Zip Configuration Archive, into your Opensolr Index Control Panel.
   
4. Once you did that, you can now go ahead and create your index in Drupal, and start indexing your data.