how do I start preparing a repertoire using openings explorer?

For starting an opining repertoire I would first make sure that you have memorized and can apply consistency in your games (based on your rapid rating you are probably close to doing that). At your level knowing a lot of theory of your particular opening is not necessary. Also getting better at tactics is also more important knowing a lot of theory for your opening. These two things will take you further right now. If you haven't read this article yet I would: https://www.chess.com/article/view/the-principles-of-the-opening

For your opening repertoire I would suggest deciding on one opening as white and two for black. For black have one to respond to e4, and one to respond to d4. You don't need one to respond to c4, or Nf3. Those are not common, and often transpose into a e4 or d4 opening (from my experience my d4 response tends to do better). You don't really need to learn more than the first 3 moves for each opening, and the basic ideas behind the opening. As you get stronger you will need to learn more opening theory, but consistently playing the same openings will help with learning all the responses, pitfalls, and traps for your opening. Hope this helps.

Edit for the first sentence. I meant memorize and can apply the opening principles.