Paper Magic is notoriously untimeable with its ten APNAP windows per turn, plus two more APNAP widows for every spell or ability, including for triggered abilities.
That’s why this is a variant. For the kitchen table. Not Comp REL.
You slam the clock once per turn cycle: after you’ve sent your attackers into the red zone.
Then blocks, cleaning up, your EOT, all that happens on your opponents time. So if they want to fetch during your EOT that’s fine because you’ve already slammed the clock so they’re doing it on their own turn.
For all rules and purposes they don’t count as AP until their untap phase starts.
Instants and activated abilities. If you want to respond on their time with a Counterspell or Giant Growth, just do it right away, don’t touch the clock, just cast the spell. But if you want to think about whether or not you want to do that, shout “okay, that goes on the stack” or “okay as soon as I get prio” or something like that while you simultaneously slap the clock so your time starts ticking down. Then once you’ve cast that spell, or decided to decline and said “it resolves”, slam the clock back.
The default in Shout ‘n Slap Magic is that you don’t get a second main phase. If you insist on one, which you have the right to do even if you don’t have abilities that make more sense in the second main like Raid, Morbid, or Survivor, shout “second main” and slap the clock right after combat is done before EOT. Slam it back after second main in done.
Same deal here, if you want to do stuff on the opponent’s upkeep you’ve got to shout ‘n slap for it. That needs to be your clock ticking as you port down all their lands.
Since you’re sending your attackers on your time and immediately slamming, if they want to Icy Manipulator or otherwise pre-combat prevent your stuff from attacking (or the opposite, force them to attack), they can retcon that and if they do you can also retcon your other attackers. Like you send two bears, they tap down one, you can then “unattack” the other too if you so wish. This isn’t a takeback, it’s, uh, let’s call it “shortcutting on assumed defaults”.
Get a clock that you can pause easily. Maybe Oracle checks and rules lookups should be on your own time (I haven’t decided yet) but huge amounts of post-block combat tricks from the attacker shouldn’t be on the defender’s time. When in doubt pause the clock or slap it to your time.