I am clearly rusty when rules are considered.

But this is my question:

How a card is worded for me to be able the break its effect into 2 pieces that I got to choose the order of resolution? And the contrary?

In order for that to be true, the card has to have two separate abilities. Look at Smokestack.

"At the beginning of your upkeep, you may put a soot counter on Smokestack.

At the beginning of each player's upkeep, that player sacrifices a permanent for each soot counter on Smokestack."

At the beginning of YOUR upkeep, BOTH of these will trigger at the same moment - so you get to choose in which order they go onto the stack.

Otherwise, if it's one long sentence, you perform it in order. For that, look at Azor's Elocutors.

"At the beginning of your upkeep, put a filibuster counter on Azor's Elocutors. Then if Azor's Elocutors has five or more filibuster counters on it, you win the game."

That ability triggers during the upkeep and puts a filibuster counter on it. THEN the ability checks to see if you win the game. If for some reason Azor's Elocutors already had 5 counters on it, you don't "win the game, then put on a counter".

Thanks. Now for the second question, there is any way to create an effect as mencioned above (as smolestack), but force the order of resolution upon the player?

Sure. Just have it all be part of the same ability. Put it in the same paragraph. Archdemon of Unx's ability:

"At the beginning of your upkeep, sacrifice a non-Zombie creature, then put a 2/2 black Zombie creature token onto the battlefield."

This says they HAVE to sacrifice a non-Zombie creature, then they get a 2/2 black Zombie. If for some reason the Archdemon of Unx was also a Zombie (Conspiracy set to zombie), they'd be unable to sacrifice a non-Zombie, but would still then put a 2/2 black Zombie onto the battlefield. If you, the designer, reversed the wording; "At the beginning of your upkeep, put a 2/2 black Zombie creature token onto the battlefield, then sacrifice a non-Zombie creature." you'd do it in THAT order instead.

If you're designing a card, I'd suggest going to the You Make the Card subforum, and post your concept in the Card Clinic - conveniently stickied to the top of the forum. There they can help you fix up the card to do what you want.

I was thinking about that, but I want to get the rules right first.

Last question, I promise. Can I respond to a card that used "then" in the middle of its effect, just before the "then" part.

No. Once a spell or ability starts resolving, it cannot be responded to.