Made HOWTO a tiny bit better.

Documentation/HOWTO-change-firmware-and-compile-it_MeesElectronics.md

@@ -91,6 +91,8 @@ release-waveshare-esp32-s3-pico:
     rm -r build
 ```
 
+The `upload-waveshare-esp32-s3-pico` section assumes the ESP32 S3 is connected to serial port /dev/ttyACM0. On your system, this could be another port. Manual change all the `/dev/ttyACM0` to the port on your system.
+
 ### Boards.h
 
 The file `Board.h` is the most important file to edit. This is the place where the supported boards are defined. For our new board, a new section has to be added. This section defines the pinout of the SPI port and if it has certain peripherals such as a screen or BLE.
@@ -297,7 +299,7 @@ The file **display.h** also has a board specific definition section. If the new
 
 The file **Power.h** must also have a board definition. And also proper definitions in **void measure_battery()** and **bool init_pmu()**. These are more elaborate and not yet fully understood, but only used when the board has a battery installed. Our example board doesn't have a battery, so we don't have to change this file. If your board has a display, make sure you make the proper changes.
 
-### Flashing the board
+### Compiling the code and flashing the board
 
 Make sure you are in the dialout group. If not:
 
@@ -307,7 +309,11 @@ If you are flashing a custom board, you will need to generate a signing key in r
 
     $ rnodeconf -k
 
-Than flash the firmware.
+Compile the code:
+
+    $ make firmware-waveshare-esp32-s3-pico
+
+Than flash the firmware:
 
     $ make upload-waveshare-esp32-s3-pico