This is probably going to be a quite big project, but Temperature measurement outside the tag - #3 by jari.isohanni should get you started.
RuuviTag is no longer officially supported by Espruino, but RuuviTag will probably run same firmware as any generic Espruino board on nRF52. You might need a wired connection to upload the firmware to RuuviTag.
If your RuuviTag board version is B8 the sensors are powered off by default by GPIO, so low-power operation should be doable. After soldering the sensor to test points under board you need to read the sensor with Espruino code and then format the advertisements according to Ruuvi data format.
Once you have the data in place Ruuvi Gateway should pick the data and deliver the data to Ruuvi Cloud, and then you can use the data in Cloud with your Ruuvi Cloud subscription.