STM32f429: problem with using external SDRAM as an additional data memory

What you are asking for is not really possible.

The internal SRAM and external SDRAM are not contiguous; their addresses are a long way apart and variables cannot simply overflow automatically from one to the other.

The correct steps for using the external memory are exactly as given in the example projects, it would be meaningless to repeat them here.

The work you have to do yourself is to decide which variables go in which memory. You can assign a variable to a section using the gcc section attribute or a similar feature of another compiler. There are examples of this in the STM32Cube package.