If you are here, means you have already enabled the MEMORY MAPPED MODE.

Now you have to create a new project, which will boot from the external Flash.

Open the system_stm32xxxx.c file, goto System_Init, and add the following code to it.

RCC->CFGR = 0; // Reset the configuration register

SCB->VTOR = QSPI_BASE; // 0x90000000

Open the Flash.ld script file, and change the FLASH address to the QSPI FLASH. Also change the size

FLASH (rx) : ORIGIN = 0x90000000, LENGTH = 16M

In the Debug configuration, goto debugger tab, and towards the bottom check the external loader.

Click scan, select the board, and click apply

The Application will run from the External memory now.