It has been quite a while since I did anything with this, but you may want to have a “programmer” as well - IIRC I used that to flash the EmonTX. Available from the OEM Shop for not-a-lot. Programmer - USB to serial UART - Shop | OpenEnergyMonitor
The thread that @borpin linked to above became hugely long, but in case it helps I did upload a How-To for Dummies guide here - Avoiding wireless connections - EmonTX serial RPiZero solution - #119 by haffle
Good Luck!