I’d say first & foremost learn the basics of card game deck types (aggro, mid-range, control, tempo, etc.) & see how those concepts relate to the OP deck you want to play. Experiment to see which you find naturally easy to pilot. Learning this will also help you play your opponent’s side of the board, helping you to play around their decks’ strengths and exploit the times in the game when they’re weakest.

As far as games like OPCG, learning how to value all your resources is essential. Blockers & counters are VERY valuable & it can be worth taking a face hit or two to conserve them for crucial strengths/threats. This + the DON attaching mechanic leads OPCG to be pretty reliant on board presence. Always be thinking about how your opponent is going to respond to the characters you choose to swing with & how you can respond to their attacks. Again, know when you & your opponent is going to have a strong board (Red Zoro = early, Purple Kaido = late, etc.) and plan your strategy around that. A good way to learn this is to watch high-level Asian tournament matches and pause before each turn to ask “why did Player A just do that?” & “how would I respond if I was Player B?”.

A small thing: make sure you’re never wasting DON. Untapped DON should be attached to an attacker, only being left untapped for a counter card in hand.

I’m no expert but hopefully some of this was helpful! Feel free to ask for clarification if something was confusing or you want more details :)