If you want to flash new firmware onto your ESP8266 module GPIO 0 has to be connected to GND during powering up the module. Normally you would do that with a jumper wire or a little switch. Of course it would be good if it could be so comfortable like with an Arduino where you just press the upload button in the IDE and the controller is flashed.
But luckily this is possible as well with the ESP. Therefore 2 additional signals from the FTDI USB Serial adapter are used – RTS and DTR. On The github project page there is a schematics how the ESP module has to be connected to the USB serial adapter. I found out that the nodemcu-devkit modules already work out of the box. They can be purchased for less then 10€ on ebay.For other modules like the ESP01 module you can build your own adapter.
The schematics is mentioning a separate 3.3v power supply. I noticed that at least the FTDI USB serial converter I am using is capable to supply enough power to flash and test a ESP01 module. Since GPIO15 is not exposed on an ESP01 module I have only connected the upper 3 10k resistors. 2 of them (REST and CH_PD) I have soldered directly to the module because they are more or less always needed. the one going from 3V3 to GPIO0 is in the adapter cable. The wiring is like this:
Make sure you set the voltage jumper to 3.3V !!!