Steps to follow
1 Stay alive while searching for a Manamorphose, a Thousand-Year Storm and Rituals to keep in hand.
2 As soon as you have enough lands and Rituals to cast Thousand-Year Storm and a Manamorphose, do so. Cast the spells drawn, add mana, keep going until your library is empty. When resolving Manamorphoses, make sure you have at least 2 in your mana pool, then add only . Make sure you empty your library with a "look at the library top, put one in hand" spell and not a "draw" spell to avoid losing the game. Then, cast all the Thousand-Year Storms while keeping at least and in your mana pool for step 3. Also, at least one Manamorphose must remain in hand for step 4.
3 Cast Fae of Wishes as a sorcery. Cast Brass's Bounty, Parallel Lives, Fae of Wishes as a creature and Teferi, Time Raveler if none is in play and +1 it. Cast Lorehold Command choosing the mode that makes Fae of Wishes gain indestructible. From now on, every spell that requires a creature target has one, including damage spells in order to not kill anyone (yet). Also chose the drain 3 mode. If applicable, cast Mastermind's Acquisition to get creature cards.
4 From your hand, set aside a single Manamorphose, 4 Mythos of Illuna, and Psychic Spiral.
5 Increase the storm count by casting, when applicable, Angel's Graces, Galvanic Discharges, Psychic Intrusion, Grapeshot and cast/retrace Raven's Crime keeping only one land in hand for step 8. Target yourself to discard nonland permanents and also Harness Infinity and Crack the Earth if applicable, then target an opponent.
6 Crack a treasure for and crack 21 treasures for . Keep this mana for steps 8 and 9. Crack treasures for symbol::U until you have as many in your mana pool as there are symbols in the remaining spells in hand. Crack the rest for .
7 From now on, whenever you would put a card from your library into your hand while your library is empty, respond with Red Sun's Zenith (with X=0). Cast all "draw" spells, and then all the "look at the library top, put one in hand", keeping a single treasure for step 7. When in need of mana, respond with a mana spell. Finish this step with at least remaining. After this step, if you haven't cast more than 10^5 spells (single casts, copies do not count), you did it wrong.
8 Retrace Raven's Crime to discard Red Sun's Zenith. Your hand should now be empty, except for possibly Time Stop. There is a very slim chance that some "draw" spells remain as well.
9 Grab the cards previously set aside. Cast all 4 Mythos of Illunas targeting Thousand-Year Storm, and target all copies (they will resolve first) on Parallel Lives.
10 Cast Psychic Spiral. The first Psychic Spiral resolves. Before the second one resolves, respond with Manamorphose. Let the copies resolve until your library is empty, then do the Red Sun's Zenith shenanigans. Still before the second Psychic Spiral resolves, go to step 4. This loop is not infinite as there is a finite amount of Psychic Spiral copies on the stack.
Neat tricks to optimize
Before the combo turn, if you intend to cast a Discover the Impossible and you are not looking for a land, know that Discover the Impossible might find a Galvanic Discharge to cast for free.
When you start the combo, you might run into difficult decisions regarding Discover the Impossible. Say you cast Pyretic Ritual, then Thousand-Year Storm, then Manamorphose: you have 4 mana in your pool and only have Discover the Impossible in hand. When casting it (and you get 2 copies), you absolutely need to find a Manamorphose. When the first one resolves, your two best options are Impulse or Pyretic Ritual. The optimal play is to cast Pyretic Ritual because if you hit a Manamorphose with Impulse, you won't be able to cast it. But if you add mana first, and then when the 2 other Discover the Impossible copies resolve you cast free digging spells, you maximize your chances of hitting a Manamorphose.
The steps 3 to 7 give a generally good casting order, but the optimal sequence highly depends on what was used before the combo. Let's look at step 6 for example. Sometimes you will create more than you'll be able to spend, and sometimes the will be the bottleneck. Here are some general ideas. Most of the time, will be the bottleneck. In this case, mana spells should be cast as late as possible, a.k.a in response to the very moment you need them. Whenever there is only 3 left respond with Manamorphose. Whenever a Manamorphose resolves, add at least 1 for the next Red Sun's Zenith. When you have played all the Manamorphoses, whenever there is only 2 left, respond with a Ritual.
There is also the option of running 2 Red Sun's Zenith in the sideboard, such that Lorehold Command can be used to create an elephant then sacrifice it to draw 2. This will only work if there is at least 2 * (storm count) * (number of Thousand-Year Storm in play) mana in the pool.
After the combo has started, splicing the Desperate Ritual is not optimal. Spending to cast Red Sun's Zenith twice will allow you to copy Desperate Ritual two more times per Thousand-Year Storm, rather than only getting the equivalent of a single copy with a splice.
Step 9 is where impossible to understand numbers are reached. If x_n := the number of Parallel Lives in play, every Mythos of Illuna copy targeting Parallel Lives can be represented as an iteration on the series x_(n+1) = x_n + 2^(x_n). If y_n := the number of Thousand-Year Storm in play, every Mythos of Illuna copy targeting Thousand-Year Storm can be represented as an iteration on the series y_(n+1) = y_n + 2^(x_n). Hint: after only 4 iterations with x_0 = 1, x_4 = a 620 digits number, you'll do a lot more iterations than that. The numbers rapidly get absurdly high.
Step 7 could be removed, and instead, if you can, finish step 8 with a Manamorphose that adds the necessay mana for the remaining steps.
Adding cards like Detection Tower if an opponent has hexproof or Shadowspear for permanents could be needed for a perfect run. Overwhelming Splendor works for creatures with shroud. Ways to get around players and non-creature permanents with shroud by choosing rather than targeting should also be considered.
