I typed all my secondaries as giant run-ons by making a list and connecting all the points with "and."
But seriously, I typed mine in paragraph form, which kind of forces me to write in complete sentences. But I've heard many others talk about writing a bulleted list. I guess it just boils down to preference and how clearly you get the information across.
I went for the "paragraph of complete sentences that tells what you did, where you did it, and how it was a meaningful experience". Some of my ECs are kind of random, so I figured it would help to flesh them out.