I and David are mentoring this project. The very first step in this process is to pass the competency test. You may find the description for the same here:
https://github.com/boostorg/boost/wiki/Boost-Google-Summer-of-Code-2018. One important thing to note with regards to competency test is the quality factor ie. quality of the code gets much higher precedence than the time taken.
Once you are done with the competency test, you can send it to us directly. When the competency test looks good enough, the next step would be to write a very detailed proposal with description of work, planning, resource etc.