You may be able to base your design on an Adafruit Huzzah (esp8266) breakout: Downloads | Adafruit HUZZAH ESP8266 breakout | Adafruit Learning System
It uses simply TX and RX and the design is intended for a 5volt programmer so you could even simplify it if you are consistently using 3.3 volts.
The disadvantage with such a simple design is that you have to manually set the device in programming mode (pressing a button).