how would one connect the MacBook 12-inch Retina to an external monitor/display?
http://store.apple.com/us/product/MJ1L2AM/A/usb-c-vga-multiport-adapter
or
http://store.apple.com/us/product/MJ1K2AM/A/usb-c-digital-av-multiport-adapter
these also work (but will not power the macbook at the same time):
https://store.google.com/product/usb_type_c_to_displayport_cable
or
http://www.monoprice.com/Product?c_id=102&cp_id=10303&cs_id=1030319&p_id=12908&seq=1&format=2
right .. so the adapter with an HDMI display ..
Yes. And if you're in the market for a monitor, make sure you get one that can achieve its full resolution over HDMI. Some monitors that have both DisplayPort and HDMI will support their full resolution on DP while only going to 1080p on HDMI.
I would get the Apple Cinema Display .. that has a HDMI input, right?
There is no Apple Cinema display with HDMI, you can use an older Apple display but it would be costly to as you would need special cables and adapters. The thunderbolt display will not work, the best options is to get a NON Apple display that can support your desired resolution over HDMI.
how would one connect the MacBook 12-inch Retina to an external monitor/display?