I think you either need to think of marketing open source pretty much exactly like any other marketing effort, or you need to not think of it as "marketing" at all. If marketing seems inappropriate, you need to think of it as how to get the word out or how to reach the people who could benefit from it or would be interested in it.
Thinking of it as marketing may be problematic because most people think of that as being about a) self promotion and b) making money. If neither of those is a comfortable fit, you still need to think about how to connect with prospective interested parties so as to further the mission.
How you do that will depend on the particulars of the project. Please give us some idea of what, if anything, you are working on. Hopefully that will lead to better answers.
Most OSS starts off with an itch to scratch and then if other people have the same itch they should be able to find you via google.