Photograph everything! You don't have to include every photo in your portfolio but you might regret not being able to include a vital piece of evidence because you didn't record it.
Record every step! Take notes along the way. Focus on what went well, what didn't work out the way you wanted and what you'd do differently next time.
Keep everything! Even if it went wrong, it's useful learning to know how to avoid or correct in the future.
Write down the ideas you have so far for your portfolio.