What we need?

Hardware

To be able to flash nodeMCU firmware into ESP 12-F we need following hardware:

ESP 12-F :-)
3.3V power supply
USB to serial converter with TTL output
3x 1kOhm resistor
10uF capacitor for at least 3.3V (5 or 6V cap recommended)
button (jumper, piece of wire, sth like that)
computer running Linux (Ubuntu 16.04 used in examples)
soldering tools, wires

Software

We need following software:

nodeMCU firmware - build your own via nodemcu-build.com - please note that your firmware needs to be smaller than flash size of your ESP
esptool - current version is 2.0
esplorer or screen

Hadrware setup

basic connections

You need to connect your ESP as on image above, so:

connect 3.3V power to Vcc (+) and GND (-)
bypass the power with the capacitor - put the cap as close to the ESP as possible
connenct RES and CH_PD pins to 3.3V via 1k resistors - on image above there is RES connected directly, but I recommend you to use 1k resistor in the same way as CH_PD in connected
connect GPIO15 to GDN via 1k resistor
connect button (jumper, sth what can be used to temporary short) to GPIO15 and GND
connect RxD, TxD, GND of USB to serial converter to TxD, RxD and GND of ESP as on image
connect USB to serial converter to your PC.

To put nodeMCU into FLASH mode you need to do following (you will need this later):

power off, press the button or somehow connect GPIO0 to GND
power on
release the button (connection on GPIO0 - GND), now you are in flash mode

Software setup

Install all the required software, make sure you have access to you serial converter device (usually something like /dev/ttyUSB0) - use following to allow access:

sudo chmod 777 /dev/ttyUSB0

We will use esptool to flash the nodeMCU firmware into ESP and esplorer or screen to check if the nodeMCU works.

Flashing the nodeMCU firmware

You need to erase the ESPs SPI flash and flash the nodeMCU firmware into it. After that you need to reset the ESP and wait until internal filesystem is formated.

In case that your ESP does not boot up, try to find out which SPI flash chip is used in your ESP and force flash parametters manually.

Erase SPI flash

Do not ommit this step, otherwise your ESP may experience problems to boot

put ESP into flash mode
erase flash using

esptool.py --port /dev/ttyUSB0 erase_flash

output

esptool.py v2.0-beta2
Connecting....
Detecting chip type... ESP8266
Uploading stub...
Running stub...
Stub running...
Erasing flash (this may take a while)...
Chip erase completed successfully in 8.4s
Hard resetting...

Flash nodeMCU firmware

The simplesy way

You can try to let esptool to autodetect all flash parametters and specify just port and filename which should be flashed. 0x0 in the command bellow means the starting address for flashing, 0x0 simply means from the beginning of the flash.

To do that, follow these steps:

put ESP into flash mode
run

esptool.py --port /dev/ttyUSB0 write_flash 0x0 <name-of-your-nodeMCU-firmware-bin-file>.bin

output will be something like:

esptool.py v2.0-beta2
Connecting....
Detecting chip type... ESP8266
Uploading stub...
Running stub...
Stub running...
Attaching SPI flash...
Configuring flash size...
Auto-detected Flash size: 4MB
Flash params set to 0x0040
Compressed 472168 bytes to 306290...
Wrote 472168 bytes (306290 compressed) at 0x00000000 in 27.5 seconds (effective 137.5 kbit/s)...
Hash of data verified.

Leaving...
Hard resetting...

DO NOT power off / reset the ESP now
open esptool and open the serial console, port speed 115200b or open serial console using screen like:

screen /dev/ttyUSB0 115200

power off ESP
power on ESP, after few seconds in your termilal (esplorer / screen) there will appear few lines of strange characters followed by message

Formatting file system. Please wait...

wait about half a minute, and you should get nodeMCU prompt

NodeMCU custom build by frightanic.com
    branch: master
    commit: 81ec3665cb5fe68eb8596612485cc206b65659c9
    SSL: true
    modules: adc,bit,enduser_setup,file,gpio,i2c,mqtt,net,node,tmr,uart,wifi,tls
 build  built on: 2017-01-15 02:24
 powered by Lua 5.1.4 on SDK 1.5.4.1(39cb9a32)
lua: cannot open init.lua
>

If it does not work / you want to optimize

In case you want to speed up the flashing process, try following

esptool.py -b 1500000 --port /dev/ttyUSB0 write_flash --verify -fm qio -ff 80m 0x0 <name-of-your-nodeMCU-firmware-bin-file>.bin

-b 1500000 will switch the speed of the serial port from default 115200 to 1500000
-fm qio force qio access to the flash, may not work on dio flash chips
-ff 80m switch the flash SPI speed to 80MHz, make sure that your flash chip can handle the speed (google datasheet), speed can be 20m, 26m, 40m, 80m, default is 40

Using command above my ESP-12F with 512KB (4mbit) flash finish flashing in 3.7seconds instead of 27.5seconds with default settings

In case that is the firmware flashing process failing, try to

try to find out which SPI flash chip is used in your ESP and try to force -ff -fm -fs parametters manually
lower serial speed using for examble -b 9600
set dio access instead of qio using -fm dio
lower flash SPI frequency using -ff 20m
check connections
read esptool documentation and instructions how to flash the nodeMCU firmware in nodeMCU documentation for more tips
make sure your power supply is powerfull enough, do not use 3.3V of the USB to serial converter

Find out which SPI chip is used in you ESP

The simplest way how to find out which SPI flash chip is used in your ESP is to look on the flash chip on your ESP (use magnifier glass if needed, it is very small), read the marking and google its datasheet. Sometimes it is not possible - for example on ESP 12-F the flash chip is under metal cover.

Use esptool to find SPI flash type in your ESP

put the ESP into flash mode (see Hardware setup)
run:

esptool.py --port /dev/ttyUSB0 flash_id

output should be something like

esptool.py v2.0-beta2
Connecting....
Detecting chip type... ESP8266
Uploading stub...
Running stub...
Stub running...
Manufacturer: e0
Device: 4016
Hard resetting...

look into flashchips.h, google datasheet

In datasheet look for:

size
SPI speed
DIO / QIO

Buger's blog

Flash nodeMCU firmware into ESP 12-F