I do not see any effect on search form default

Have you set up and connected an elasticsearch cluster?

Yes I have installed on the server:
https://www.elasticsearch.org/overview/elkdownloads/

That’s the link where you download it, yes.

Are you able to access your cluster from a url? For example:

https://your_url:9200

I have the same problem, I am able to access the cluster. Could anyone tell me how to check if it’s running?

Go to your server root and enter
curl -XPUT https://localhost:9200/myindexname/
Now go to server setting in plugin of elastic search.
And put
Server URL
https://localhost:9200/
Index Name
myindexname
Read Timeout
5
Write Timeout
300
and save it.
In WordPress Integration
Enable Search:tick right.
and save it
Now go to
Manage Index
Click on wipe data
Wipe Data
then click on
Re-index Data
it will take some time
and after click on save changes.
Now go to your site and search fantastic with elastic.

Thanks for the reply.
I have done all this. Re-indexing has been done. But how do I check if the results coming are from wordpress default search or from elastic search?

I’ve created an issue on github to address detecting if the index exists and attempting to create it. Should probably have clearer instructions for first time users:

https://github.com/parisholley/wordpress-fantastic-elasticsearch/issues/109