just connect them together with two resistors like this page says:

http://michaelteeuw.nl/post/113107637802/esp8266-no-strings-attached

    ESP8266’s VCC is connected to a 3v3 power supply.
    ESP8266’s GND is connected to the ground.
    ESP8266’s RXD is connected to Arduino’s pin 4 using a voltage divider
    (330Ω to GND, 170Ω to pin 4).
    ESP8266’s TXD is connected to Arduino’s pin 5 (The Arduino can handle
    3v3 just fine.)
    ESP8266’s CH_PD is connected to VCC using a 10k pull up resistor.
    (This will enable the wifi module.)

On Fri, 5 Feb 2016, robb wrote:

i've discovered 5v (arduino) to 3.3v (esp8266) serial requires a logic level shifter.
i have a max3243 with no idea how to use it.

anyone know how to use a Multichannel RS-232 Line Driver/Receiver or
know a good circuit schematic that converts 5v-3.3v logic?

thx
~r