You'll have to familiarize yourself with Rule Machine in general. The online documentation is here: Rule 5.1 | Hubitat Documentation
It sounds like temperature readings from some sensor would be your triggers and expressions (for deciding which threshold was crossed and therefore what command needs to be sent), and sending the saved RF command would be the actions.
Once you get to the point of adding the 'action' to your rule, you have to use the custom command step, which looks like this:
