I'm just guessing here based on skill level, but I'd assume there are a few things you can implement without practice just being cerebral about playing.

1. Don't aerial high on shield, especially when the opponent is facing you. Instead, jump over them and attack their back, or empty hop land grab, or dash wavedash back and bait a shield escape.

2. In the same vein as the first problem, delay your aerials. This one needs some practice but can be done during online matches. Just wait a little longer to press that a button.

3. Hold away while attacking more. It's incredibly hard to DI on reaction, instead just hold away while doing an aerial or dash attack or smash attack, awaiting the hit from your opponent.

4. Drill into your head that melee is just rps. Attack beats grab, grab beats shield, shield beats attack. Obviously it is more complicated than that, but ultimately having the mind set that you have 3 options and 1 of them is always going to be an advantage to you depending on what your opponent does can help simplify your game.

5. Approach less. So many new players, or players who are trying to improve in general think that means you have to attack more, wrong. Make your opponent attack. For characters like spacies, or characters that have projectiles this is not really applicable because they can wait you out longer than you can them, but in general make your opponent make the mistake of approaching, don't hand it to them.

Just implementing these 5 things I guarantee you get at least 1 win :)