Help designing a Bluetooth controlled square wave generator
Start with a commercial Bluetooth module that encapsulates the stack, read and understand all the application notes and datasheet. It will act more-or-less as a wireless serial port.

Implement the serial-port-controlled square wave generator with whatever technology is appropriate for the performance requirements- microcontroller with on-board peripherals, aux DDS or whatever.

If you are planning to sell this, read up on the license and other fees to use BT.