Ask HN: How do I go about open sourcing a project?

I've been working on a startup idea for over a year, and I have a fully functional prototype. I've decided that I don't want to pursue the idea, but I don't want all of my work to go down the drain. I do mentoring for startups often and find that there is a lot of reinventing the wheel when it comes to getting an MVP going. It seems that the vast majority of the code that I've written could be repurposed for almost any other web-based startup in a couple weeks. How do I find out whether there's any demand for this and if so are there any guides to properly open sourcing a project? I don't want to just dump the code out into the ether.