The process of adding SSL to a web site requires a few steps.

Contact your web host and ask for their help as they will have their own method of adding SSL to a web site.

This is what I usually do with my clients or my own WordPress web sites.
– Check in with the web host in regards to the process to setup SSL.
– Login to the WordPress site and go to the backend. Then to Settings -> General then change the address:
WordPress Address (URL) and Site Address (URL) from http:// to https://
One will then have to login to the site again. Clear the cache to see the new result.

– Install the WordPress plugin Better Search Replace to locate the leftovers of http.
Search for http:// and replace it with https:// . Select all the tables listed. Do a dry run first then uncheck the dry run to replace all the instances of http:// with https:// .

If the site is still not secure with mixed content then check the site here:
https://www.whynopadlock.com

An example. The site I tested used the plugin Smush to make images smaller. 3 images were not added http to, so I deactivated Smush and used Better Search and Replace to see if the 3 images would be found. They were and I adjusted them from http to https. I then reactivated the Smush plugin again. Checked the front page and noticed the green padlock. I then also checked the site again with whynopadlock.com. This time the site had become secure.

NB!
If an image it not secure you might need to to the page where it is used and manually change the image url from http to https.

Adjusting http links manually

Right click anywhere on the frontend of the site and select Inspect.
If there is a red circle with a x in it. Click it. It will show the errors with the mixed content.
The links that use http instead of https.
On the right side it might say (index):1 etc. Click it. It will show the location of the error.
I next went to the backend. In Appearance -> Theme Editor. I selected the stylesheet style.css.
Then searched for http:// and changed all I found to https://
Next I went to functions.php and also changed all of them from http:// to https://

I went to the frontend again to see if there were any additional errors I found one more in a js file so I went to the backend and found the file mentioned and once again changed to https://
When or if the theme has an update one might need to go back and adjust.

A few more detailed resources:

Check your site with SSL labs test: https://www.ssllabs.com/ssltest/

enshrined.co.uk/2016/02/24/moving-wordpress-onto-https/
designmodo.com/wordpress-https/

An example to add SSL can be through cPanel: archhosting.net/wiki/SSL_Certificate

Additional resources:
whynopadlock.com