You are able to assign the SSL certificate to your domain in the c-Panel. But please take some time and take a look at the description of your web hosting company. Your domain is for example: domain.com. But if you are still using the www.domain.com, then you have to know the www is already a subdomain and not all provider include the www into the free certificate! If you assigned the certificate to your domain, it will take some time until it is available.
Try to manually call your website in the web browser with https://domain.com (or https://www.domain.com if you used this version before) and if this works, the activation process is done.

Switch the RapidWeaver project to use https

Now we take a look into our RapidWeaver project. It doesn't matter if you use a theme or you have built your site with Foundation, Foundry or other Framworks. The steps are the same.

Head over to Settings/General
Project/SettingsChange the Web Address from http:// to https://

change the URL to https
Changing the Web Address will change every page link in the project from http:// to https://
This was easy! Thanks Rapidweaver!

Change custom URL integration

We only have to look at links we filled in manually.
Change every manually added link in the project. Check your website site by site for this task. Sometimes you didn't know you wrote a link by yourself for some reason, but right know you have to find the URL and change it. Warehoused images and Fonts: Change the URL. Check the code you pasted somewhere in your project. Check twice.

Change http to https by hand
Publish the project and test the URL with https://www.domain.com in your browser. If you can see a lock icon before your domain name, this page is secured. Congrats. Check all other pages and if you will not see the lock icon, it could be you missed a http:// somewhere on this page.

Redirect http:// to https://

If you are happy with lock icon at your domain name (looks great), you should check your site for a redirection of non-secured URLs to secured ones.
Some web hoster provide a solution to automatically do this for you. This is a great way to make your life easier. If you still reach the http:// page in your browser, then you have to enable a re-direction on your server. There are many different server configurations and possibilities to enable this. This is an example which will work for most common sites:
In the root directory of your website on your ftp server, there can already be a file named .htaccess
The dot before the name does mean it is invisible. First check your FTP client to show hidden files to be  sure you can see all files. If it does not exist, we have to edit a text file (plain text. No RTF) and upload it to our server. Rename the text file to .htaccess after uploading.

Sample for a redirect of http to https

RewriteEngine on
RewriteCond %{HTTPS} !on
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

Check your website (disable the cache in your browser just to be sure) for perfect redirections from http to https.
More infos about the htaccess file: http://www.htaccess-guide.com.
Note: If you are using the Google Search Console (Formerly Google Webmaster Tools), then you have to add a new property with the https:// prefix. Thank you Paul for this addition.