I didn't grow up with many male role models so it was very difficult for me when it came time to have mature relationships. I had to learn things the hard way and I've had to consistently work on myself and my relationship to keep things running smoothly. My spouse and I have been together for about a decade now and we're still together thanks to hard work. I owe a lot of that to her. While I prefer to think of myself as God-like, the truth is that we never would have made it this far without her infinite patience and understanding. My first piece of advice to you is to get extremely lucky and find a good partner who is willing to wait with you while you learn these skills.
For the sake of ease, I will use 'she' when referring to a potential partner. That being said, if you would rather have a male partner replace she with he.
She is important. I was going to write "make her feel important" but "make her important" rings more true. Because it is. She is important. You have to remember that and use it to guide your actions that you need to take. When you come home from work and she tells you about her day, pay attention to her. Her words and feelings are more important than your phone screen. Listen to her and make sure she is aware that you care. Make sure she knows that she has your undivided attention as you speak to her. Look her in the eye, nod along, ask follow-up questions. Don't interrupt her. Do things for her that require effort. Sometimes in relationships we treat it like some kind of game with points. While flowers are nice, they don't take much effort. You might get some "points" for them but it is so much more meaningful if you can put in real work and effort. Show her how much she means to you. Even after years you need to find a way to tell her, if not show her, "You are important to me." This is important.
Second, make her feel good. Learn how to eat good pussy. I'm not going to bother using euphemisms because it is what it is. Eat pussy and love it. Get good at it. Make it a hobby that you actively pursue improving. Get your face don't there and get your woman off. Stay down there until she does. I don't care if your jaw hurts or you can't breathe or your fingers are tired. Some nights she'll be quicker than others but keep at it even when it's difficult. Trust me, if you give good head she'll want to stick around even when things get bad.
Third, maintain yourself. Don't be that guy who gets a Dad bod before he even becomes a dad. Get a gym pass ignore the pizza and get jacked. She's your partner. You care how she looks and you can be damn certain she cares how you look. So keep it up and don't get lazy just because she settled for you. Be a trophy husband.
Fourth, take her side. You are in this together. She will be wrong sometimes. You can give her your opinion, your advice, but ultimately there will be times when she is about to do something outlandish and wrong. Stick with her and go for the ride. She needs to know that you'll be there for her even when times are difficult. You have to be someone she can rely on, not just trust. When things get tough for her it is your responsibility to be there with her. Have her back.
Fifth, don't spend all of your time with her. Take time for yourself with the boys. Have Guy Nights. Have hobbies outside of her and keep up with them. You are your own person. You can have a relationship and maintain autonomy. You are not some weird pair that can only exist together.
Lastly, always kiss her with passion. Don't do anything half-assed but especially don't kiss your wife half-assed. Do it right, with all of you. Do it like you mean it, every time. A kiss between you and your wife is the easiest way to say "I love you" and don't you DARE love her lazy. Get out there and show her how much you care. Tell her. Plant a big old kiss on her and mean it!
And do almost the exact opposite of all this to actually find the girl in the first place ;)