WELCOME TO PART 2!!!
The Main Character
First, you need to say why this plane is important. Is there a Planeswalker who was born here? Who came here on a quest or by accident? Why are we following this story. Theros was mostly based around Elspeth, Khans around Sorin. If this is an existing Planeswalker, make them special. Make them go through a change or great development that maybe goes far enough to change their colors, or at least their abilities. If it is a new character, this process becomes a lot more in depth. Let's start, answer these questions and build the story and character around them.
Gender, "species," race and traits. (You should have multiple cards based off them so pick one you can get art for.)
Are they a Planeswalker?
On what plane were they born?
What faction where were they born and raised?
Who raised them, what made them special?
What is their personality?
If they are a Planeswalker, how did their spark ignite? How did this change their personality?
What brought them to the beginning of the quest.
Who are their friends and family?
Questions that will be answered after the previous ones.
What colors are they?
What do their abilities depend upon?
What do they care about? What do they seek?
What do they rely on for power and support?
Who/What is this quest for? What will they gain.
How do they survive their quest?
What do they get throughout the quest? (Weapons, armor, knowledge, skill, friends, abilities, spells)
Once you have made the character, make the card. It should be the first card you make. Post the lore based off those questions into the comments of that card. NO ONE ELSE SHOULD MAKE CARDS. This is the only card made as of now, and it will be until the count if complete.
This is a much simpler process. Describe the same questions as before, but not as in depth. Then add:
What is their connection to the world?
What is their position?
How do they connect to the main story and main character? If they don't, how is their story important. These legendaries should be humanoid, in someway.
Example: Forgotten Dragons; we give lore to the 5 new would-be-khans, but much less to the 5 new Dragonlords.
There should be one of these special people for each faction. These people could be the leader, a low-life whose a friend with the hero, or someone the hero met. But as of now only 1 per faction.
Next, you should fill in the spots with the people who are important to the world but not the story. If one faction doesn't have a ruler's lore yet, make that as their second legendary. Per set I like to have 10-16 legendary creatures, and 2 Planeswalkers (unless the lore changes that, like Hellfall did.)
Now that you have your main characters and people who are important to her at the beginning of during the story, begin it. When you made the characters you should of based them off of where they fit into the story, so just fill in the blanks. If Sorin wants to find Ugin as his final quest, and he meets Narset along the way, fill in the spaces in between those to major plot lines. Add in encounters with normal creatures and travels, and remember to include all the characters you made based on where they go in the story.
Since all of the Legendary creatures are at important times in the story (that's the only reason you know their names vs. just "Hardened Barbarian" as a name. These points is where they gain knowledge or item or spell or ability of some sort. If you make a transforming Planeswalker like in Origins, they will be sparked or "flip" at one of these times. They will gain their special items that will become cards but only effect the story since afterwords.
You should never make the Side quest unless it effects someone the main character met, or if it effects the world in a way that changes the side quest. The side story shouldn't effect the cards that much UNLESS YOU ARE MAKING A BLOCK. When Narset dies, nothing happens in Khans and becuase of Fate Reforged time traveling shenanigans nothing happens overall.
Remember Side Story isn't important unless in leads to effecting the main story. Characters like clan leaders should be made even if you never meat them, just because they're important to the plane. EXAMPLE: Sorin came to Khans just to find and then mourn Ugin's death. But nothing else happened. Include characters like that, don't put them on an adventure.
Join me next time to learn how to make custom mechanics that fit the set. Next time will be highly example based.