The old days, point all traffic to your server and 301 it to another location. The rise of "encrypt all the things" makes this a bit harder, as every source requires a SSL certificate to be able to redirect properly, otherwise you'll get the gnarly SSL errors.
This completely free service makes creating redirects a breeze!Start creating redirects Login
Add your source(s), for instance my-old-site.com. These will be redirected to your new destination.
The sources which you want to redirect require a DNS change. We'll wait a bit until it's fully propagated.
DNS all set? We'll ask the folks over at LetsEncrypt for a SSL certificate and install it automatically.
All source traffic is now redirected to the destination!