The LCD screen communicates to the Cortex Microcontroller via a UART cable. This communication needs to be initialized in init.c:

void initialize() {
  // ...
  // Select "uart1" or "uart2" as appropriate
  // ...

And in any function you can do the following:

// Print formatted text to LCD (line #1) like printf
// supposing that "x" is an existing integer variable
lcdPrint(uart1, 1, "X is %d", x);
// Print plain text to LCD (much faster than lcdPrint)
lcdSetText(uart1, 2, "Hello World");
// Print current battery voltage
lcdPrint(uart1, 1, "Batt: %1.3f V", (double)powerLevelMain() / 1000);
// ...