How to flash a NodeMCU (ESP8266) with an external FTDI programmer?
Assuming your FTDI Basic is the 3.3V version, then yeah that will work fine provided you that manually reset the board yourself and enter the boot-loader using the buttons.
If you're independently powering the boards you may want to insert some series resistance in the RX and TX connections in case your signal receiving end is not powered while the sending side is.  In short you don't want the TX signal from one of your devices (boards) trying to power the receiving device (board) through its protection/clamp diodes.
In regard to comment about resistor value:
The value of the resistor depends on what you can get away with in terms of its effect on the signal and how much current is allowed through the clamp diodes on the other board.  A couple of Kbyte is probably sufficient, better than nothing in any case. Use a bit more if you can and still get a reliable signal through.  If you have the boards both powered before you connect them and you're careful about connecting them, then there's no opportunity for that.  It may also just be that they're robust enough that you don't need them at all.  I usually put some in though.  The second reason which I didn't state above is just that it protects you from being outright stupid and accidentally connecting them to the wrong thing; wrong signal, or to VCC or GND on the other side.