For example, after 1.e4, don't spend any time studying 1...h6. Black has wasted a tempo by ignoring development, ignoring the center, and weakening the Kingside without knowing how play will evolve there. If you play on general principles, you'll get a very good game.
After 1.e4, the responses should include at least 1...e5, 1...e6, 1...c5, 1...c6, 1...d5, 1...d6, 1...Nf6, and 1...g6. There are a few more moves that might be playable, but eight responses is already a lot.
For each of these moves, choose ONE move that you want to learn more deeply. For example, after 1.e4 e5, you could choose ONE of 2.Nf3, 2.f4, 2.Bc4, or 2.Nc3.
After 1.d4, the responses would be 1...d5, 1...Nf6, 1...f5, 1...c5, 1...g6, 1...e6, and 1...Nc6. Again, choose one move to learn in depth. Against 1..d5 you should choose one of 2.c4, 2.Nf3, 2.e3, or 2.Bf4.
Since transpositions are more important to 1.d4 openings than to 1.e4 openings, you should choose the most popular responses. For example, choose 1.d4 d5 2.c4 over 1.d4 d5 2.Nf3 until you start to recognize how the different responses affect transpositions.
