How do I redirect from http to https on apache2.

I need to redirect a website already up and running on a digital ocean server from http://www.example.com to https://www.example.com. I’ve been working on it all day and have been able to generate an ssl certificate from lets encrypt and have stored it in a specific file in my directory. I know I need to give the server the location of the certificate but I dont know where I have to type that in. I tried following a tutorial https://www.digicert.com/ssl-certificate-installation-ubuntu-server-with-apache2.htm but that doesnt seem to work either. On running a config test, I get an error saying that snakeoil.key (private key) file does not exist. Although when I sudo nano that file, I can see the key (it does exist!) I dont know what to do here anymore. Any help will be greatly appreciated. Thanks!