In collecting foundational evidence, we're talking about doing your homework and catching up on your knowledge of what's happening in the field. If you're looking to make improvements in your library program and have no idea where to start, looking at foundational evidence is a good beginning. In academic terms, you're basically going to go back to school and start by conducting a literature review.
This process is going to look awfully familiar to many of you as you're probably teaching it to your students, whether it's Guided Inquiry Design, the Big6, or a model of research and inquiry that you developed specifically for your students, collecting foundational evidence is roughly the same process. Start with your question of inquiry, conduct a search from materials, read up on what you find, follow the leads provided by others, and compile everything into something useful for your work.
The next step is looking for process evidence. Collecting process evidence is about going beyond the literature that's currently available on the field. In reviewing and using process evidence, you could be gathering information from attending conference presentations, viewing online workshops from your favorite publications, scouring social media for current trends and ideas, and any number of other places.
Rather than using theories that have withstood the test of time, the evidence you collect about what's happening in practice could be a little trendier, a little experimental, and perhaps even a little controversial. This isn't to say that process evidence is lesser; it's merely different. You're using the know-how of current practice mixed in with some validation from past practice and using it to shape future practice.