How to apologise to parents properly?

How does one apologise sincerely to a partners parents for hurt or issues they had caused?

How does one show sincere regret and remorse and pledge never to repeat them again?

How does one show and express true sorryness to partners parents?

what should 1 say? how should 1 act? what gifts items etc should 1 bring? what tone should 1 have? what clothes should 1 wear? etc etc etc

...there was this other, similar thread

http://www.japan-guide.com/forum/quereadisplay.html?0+8338

...your partner?

Usually, and universally, the best way to apologize to someone is to start by calming down.

Because quite often people make mistakes that require apology from not being able to calm down. If you go to someone's parents with a kind of mind that makes you write "what should 1 say? how should 1 act? what gifts items etc should 1 bring? what tone should 1 have? what clothes should 1 wear? etc etc etc ", they would just think, "This guy's gonna do it again."

On the other hand, if you calm down, you will eventually discover for yourself what you need to do and how you should do it.

Uco's advice is excellent. One small thing I would like to add is that sincerity is expressed in your tone, actions, and stance more than in your choice of words or phrasing.

I am calm and rational now. So what to do from now?

I was going to go dressed well but not fashionable. Be somber and show remorse. Bring a box of sweets as a gift. Apologise for the bad japanese and or misunderstandings in culture.
Then apologise for everything. Accept responsibility. And promise never to do it again. Say sorry gomen nasai sumimasen deshita etc and 1 deep bow with HONTO NI GOMEN NASAI URESHITE KUDASAI

and then sit and hear them out and just accept it all.

However on my part should I bother explaining anything? or just let them empty out etc and I admit i was wrong and thats that? should i say eg though its not a reason to have done what i did i was under a lot of stress from family, work, deaths etc and emotional issues between us.

Think maybe its wisest to just accept the blame even if im wrong and apologise profusely and promise never to repeat it again. However if they want reasons for why i did such and such then what? should I give any reasons or just say im wrong? If i keep saying im wrong etc might make me look like an arrogant evil prick rather than a human being with stress issues and feelings etc who though was wrong is still human.

Also should I point out the sacrifices I have made to be with her? Time, money, not leaving japan for her, changing plans to accomodate her, washing and cleaning her, taking care of her, building house etc???

As was posted before, they will be able to tell if you are sincere by your tone and actions, rather than just your words.

Also, it's better just to apologize. Going into why, what sacrifices you're making, etc., etc., sounds more like you're not sincere about your apology, and more interested in trying to make them believe it was ok that you did what you did...which will backfire on you.

In my experience, when you don't know what to say, a "sumimasen" (or maybe a moshiwake arimasen is appropriate) by itself and a humble attitude go a lot further than saying anything additional in your defense. NOT saying anything more shows that whatever your reason is, it's not a good enough excuse for what you did, and that's a more sincere apology. Plus, if you're not a native speaker, it will prevent you from getting into even more hot water.

How long should 1 wait to apologise though? Though I wanted to do it now I was told to wait since the parents are too pissed at the moment and give it more time.

Then is no hard rules about what you should say/when you should do what :) I think it's better to talk through with your partner. She knows her own parents well. If it's her who suggested that you wait, then you should wait.

Whether you should explain anything depends on what they want to hear (and what your partner suggests). If they want to know, they will ask. If you are to explain, do not forget to add "I am fully aware that those are only excuses..." type of phrase ("iiwake ni shika naranai no wa juubun wakatte imasu ga...")

Also should I point out the sacrifices I have made to be with her?
Why would you want to talk about something like that when you go apologize? Sacrifices are only relevant as background for the stress you've been under, at most. If you talk about those, now that could make you sound like an arrogant prick.

Should this question considered to be a cultural thing? I don't think so. And they're just telling you to wait, because it doesn't work to speak to someone who is all upset. There is no fixed answer as to how long a person should take to be calmed down. (You should know all that if you're going to apologize to someone.)