How do I install a new thermostat to the unmarked wires that were left sticking out of the wall?

There should be 4 wires, red is hot and hooks to R terminal, white is heat and hooks to W terminal, Green is blower and hooks to G terminal, last color can be blue or yellow which is a/c and it hooks to Y terminal

Thank you SO much. I spent an hour reading "how to install a new thermostat" articles and never saw one that mentioned the 'alphabet'.

How many wires are sticking out the wall and what colors are they?

Worth. I agree completely. However, we really need to know what type of system they have.

You need to know, before you go to hooking up wires, 1st kill the breaker to the furnace,, then at the furnace end see what each wire and color are hooked to

Kjo, it does not work that way. You tell us how many wires and the colors and then we can tell you how to connect them to a stat. https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-faq-northerheat-58313.html Tell us the type and brand or model of your frnace and if it has AC, that would help