Either use the pre-generated projects, or:
git clone https://github.com/ianpatt/common
git clone https://github.com/ianpatt/skse64
cmake -B common/build -S common -DCMAKE_INSTALL_PREFIX=extern common
cmake --build common/build --config Release --target install
cmake -B skse64/build -S skse64 -DCMAKE_INSTALL_PREFIX=extern skse64
cmake --build skse64/build --config Release