We create custom index for the below reasons:

1. Fast responses.
2. Index only content items which are required.
3. Can change the indexing strategies based on the requirement.
4. Manual indexing becomes easy because we don’t need to index whole tree to accommodate new items.

Steps to create and index the new/custom search index

1. Go to the SOLR folder (in my case: D:\Sitecore Demo\solr-7.7.2)
   
2. Navigate to Solr folder (D:\Sitecore Demo\solr-7.7.2\server\solr) where all cores are listed.
   
3. Copy the “SitecoreDemo_web_index” folder and paste. Rename the new folder as “SitecoreDemo_custom_index”.
   
4. Access the folder.
   
5. Edit the core.properties file and change the name attribute to “SitecoreDemo_custom_index”.
   
6. Go to Solr services in services.msc (in my case: SOLR7.2). Restart the service.
   
7. We are almost done, let’s create the config file for our new Core.
8. Go to the App_Config folder in our Sitecore instance (in my case : C:\inetpub\wwwroot\SitecoreDemo.sc\App_Config)
   
9. Navigate to Content Search folder inside Sitecore folder (C:\inetpub\wwwroot\SitecoreDemo.sc\App_Config\Sitecore\ContentSearch)
   
10. Copy the “Sitecore.ContentSearch.Solr.Index.Web.config” file and paste it. Rename the new file to “Sitecore.ContentSearch.Solr.Index.Custom.config”
    
11. Edit the file and the values highlighted below:
    
12. Save the file.
13. Recycle the app-pool.
14. Access the control panel in Sitecore >> Indexing Manager.
    
15. The new core is visible in the indexing manager and we can rebuild the indexes for the same.
    

Thank you.. Keep Learning.. Keep Sitecoring.. 🙂