Building a portfolio helps you keep track of what you are learning, what you are strong at, and what you still need to do. It makes sense to do this as you go along rather than only putting it together at the end of Div II. At the end it is too late to realize you should have taken a methods course or worked on your analytical writing, or added a media course to help you be more creative in how you presented your theoretical learning, etc.
Start putting your important work in a binder as soon as you can – kind of like keeping an inventory of your work. Add your evals and self evals after each semester. Look over them for patterns to see what you should be working on.
And importantly, bring your portfolio to your committee chair and/or whole committee each semester to show them how you are doing. It helps them advise you when they see what you are already good at.
One other benefit of doing this is you won’t be scrambling to build your Div II portfolio when you really want to focus on the transition to Div III at the end of Div II.