Use brew to install certbot:
brew install certbot
run certbot in manual mode
sudo certbot certonly --manual
Enter your domain name(s) using comas to separate them: example.com, www.example.com
Say yes when asked if you are OK with your IP being logged. You should then see something like that:
Make sure your web server displays the following content at http://example.com/.well-known/acme-challenge/pTcxG03iF-qLqp_ysuperlongcode before continuing: pTcxG03iF-qLqp_superlongcode.u_superlongCoDe-fPA7NEJ-SuperLongCode
Log in to your arvixe server, for instance using ssh but using cpanel file manager works too. Create a folder named ‘.well-known’ at the root of your web directory (typically /home/YOURARVIXEUSERNAME/public_html), then create a subdirectory named ‘acme-challenge’. you should now be able to navigate to it:
create a file named pTcxG03iF-qLqp_ysuperlongcode (name as provided by certbot), and in it paste the text provided by certbot (pTcxG03iF-qLqp_superlongcode.u_superlongCoDe-fPA7NEJ-SuperLongCode). Go back to certbot and press Enter. Your cert should now be created.
Go to cpanel, security section, click on SSL/TLS:
In ‘Install and Manage SSL for your site (HTTPS)’ section, click on ‘Manage SSL sites’.
Choose your domain.
In ‘Certificate: (CRT)’ paste the information that you can find in cert.pem file (should be in: /etc/letsencrypt/live/example.com/cert.pem).
In ‘Private Key (KEY)’ section paste the information in privkey.pem (should be in: /etc/letsencrypt/live/example.com/privkey.pem).
Press the ‘install certificate’ button, et voila!
If you want to automate it, you may want to have a look to that page.