Prerequisite

Follow Raspberry Pi's "Getting Started with Pico" document to setup a proper development environment on your host PC (the machine that will build your project). Notice that the setup instructions are a little different for building on a Windows PC.

Either set an environment variable named PICO_SDK_PATH that points to your local clone of the pico-sdk or put the pico-sdk next to the RF24 folder or next to the folder containing your project using the RF24 library:

path/to/github/repos/
    pico-sdk/
    RF24/
    my_rf24_project/

Alternatively, the RF24 repository (and optionally the RF24Network and RF24Mesh repositories) can be included into your project's "lib" folder as copies or git submodules. For more detail, see the below instructions to incorporate RF24 libs into your project.

Building the RF24 examples for the Pico SDK

Each example imports a set of constants (CE_PIN, CSN_PIN, IRQ_PIN) from the examples_pico/defaultPins.h file. Default values for SPI pins (SCK, MOSI, MISO) come from the Pico SDK repository's pico-sdk/src/boards/include/boards/<board_name>.h files

Create a "build" directory in the RF24 repository's root directory and navigate to it:
cd RF24

mkdir build

cd build
Configure CMake for your desired build type and specific RP2040-based board
cmake ../examples_pico -DCMAKE_BUILD_TYPE=Release -DPICO_BOARD=pico

Or if building on a Windows PC:
cmake -G "NMake Makefiles" ../examples_pico -DCMAKE_BUILD_TYPE=Release -DPICO_BOARD=pico

The supported RP2040-based boards are listed in header files in the Pico SDK repository's pico-sdk/src/boards/include/boards/<board_name>.h files. If the -DPICO_BOARD option is not specified, then the Pico SDK will default to building for the Raspberry Pi Pico board.
Build the examples using the CMakeLists.txt file located in the RF24/examples_pico directory.
cmake --build . --config Release

Notice we specified the build type again using the --config option.

Note
If you see an error stating "'PICO_DEFAULT_SPI_SCK_PIN' was not declared in this scope", then it means the board you selected with the -DPICO_BOARD option (in step 2) does not have a default set of SPI pins defined for it. To workaround this error, see the below instructions to use different pins for the SPI bus.

Warning
If doing consecutive build attempts, it is strongly encouraged to delete all files in the build directory before re-attempting to build the project.

Incorporating RF24 libs into your project

In order to use the RF24 libraries in your RP2040 based project:

Make a copy of the RF24 library (and optionally RF24Network and RF24Mesh libraries) in a "lib" directory located in your project's root directory.
```
  path/to/my/project/
      lib/
          RF24/
          RF24Network/
          RF24Mesh/
      src/
          CMakeLists.txt
          ...
```
Alternatively you can add the RF24* repositories as git submodules.
Include their CMakeLists.txt files from the RF24 libraries in your project's top-level CMakeLists.txt file (usually located in the "src" directory). The following snippet assumes that your project's "src" directory is on the same level as the previously mentioned "lib" directory.
include(../lib/RF24/CMakeLists.txt)

include(../lib/RF24Network/CMakeLists.txt)

include(../lib/RF24Mesh/CMakeLists.txt)
In the same CMakeLists.txt file from step 2, add the RF24 libraries into the target_link_libraries configuration:
target_link_libraries(${CMAKE_PROJECT_NAME}

# ... Your project's other libraries ...

RF24

RF24Network

RF24Mesh

)

If you are using tinyUSB, this line (or similar) should already exist:
target_include_directories(${CMAKE_PROJECT_NAME} PRIVATE ${CMAKE_CURRENT_LIST_DIR})
Finally, remember to include the necessary RF24* libraries' header files in your project's source code where applicable.
#include <RF24.h>

#include <RF24Network.h>

#include <RF24Mesh.h>

RF24.h

Note
Any of the project's source file(s) that includes any of RF24* libraries must be a C++ file. It is highly recommended to use the .cpp file extension for such project source files.

Using different pins for the SPI bus

Initially (without modification), the SPI bus uses the default pins defined in the Pico SDK repository's pico-sdk/src/boards/include/boards/<board_name>.h files. However, there may be some boards that do not define the necessary pins to use as defaults. This can be rememdied using either project source code or build-time configuration.

Warning: There is no support for software driven SPI on RP2040 based boards at this time. If someone is so inclined to implement this using the Pico SDK's PIO (Programable Input Output) feature, please submit an issue or pull request to the RF24 repository.

Note: Before deciding what pins to use for the SPI bus, review the GPIO pins' "Function Select Table" in the Pico SDK documentation. There are essentially 2 SPI buses with multiple pin options for each.

Project Source code option

This option is the most reliable and flexible. It involves calling SPI.begin() and then passing the SPI object to RF24::begin(_SPI*).

#include <RF24.h>
 
RF24 radio(7, 8); // pin numbers connected to the radio's CE and CSN pins (respectively)
 
int main()
{
    // again please review the GPIO pins' "Function Select Table" in the Pico SDK docs
    spi.begin(spi0, 2, 3, 4); // spi0 or spi1 bus, SCK, TX, RX
 
    if (!radio.begin(&spi)) {
        printf("Radio hardware is not responding!\n");
    }
    // continue with program as normal ...
}

Build-time configuration option

To specify the default SPI pins used at build time, you can use either:

declare these pins in the CMakeLists.txt file
target_compile_definitions(${CMAKE_PROJECT_NAME}

PUBLIC PICO_DEFAULT_SPI=0 # can only be 0 or 1 (as in `spi0` or `spi1`)

PUBLIC PICO_DEFAULT_SPI_SCK_PIN=2 # depends on which SPI bus (0 or 1) is being used

PUBLIC PICO_DEFAULT_SPI_TX_PIN=3 # depends on which SPI bus (0 or 1) is being used

PUBLIC PICO_DEFAULT_SPI_RX_PIN=4 # depends on which SPI bus (0 or 1) is being used

)
additional command line arguments
cmake --build . --config Release -DPICO_DEFAULT_SPI=0 -DPICO_DEFAULT_SPI_SCK_PIN=2 -DPICO_DEFAULT_SPI_TX_PIN=3 -DPICO_DEFAULT_SPI_RX_PIN=4