These are the steps that I use to craft an idea.
In general, when I follow the steps in this order, the cards are the best.
In this example I'll use my newest card idea, Mirror Golem.
Step 1 - Concept
The first step is to think of a concept. This is basically an idea of a single effect that is unique and different.
In this case, the first thing I thought of was the "Reflective" passive. When your concept is unique and interesting, and provides something new to the game, people will like it.
Step 2 - Delivery
There are several ways to bring the "Reflective" mechanic in the game. The goal of figuring out your delivery is finding out which card type will create the least redundancy in the game.
I had 3 options:
Spell: A spell that would give the Reflective effect to a creature. I chose to avoid this route because it would be too similar to Quintessence.
Permanent: A permanent that grants your creatures Reflective. I chose to avoid this because it is redundant to Cloak and is more difficult to balance.
Creature: A creature that already has Reflective. I chose this route because it gave me the most options and lets me fill a unique niche.
General Pros and Cons of the three routes:
Spell: Spells are the most versatile route. When a spell does something, you can use it on anything, and it has the bonus of being unstoppable except by Silence. Most of my favourite ideas are spells.
Permanent: Permanents are remains in play abilities. They generally affect the whole field. They are also very fragile to any kind of permanent control. However, many abilities may become overpowered if they affect the whole field indefinitely, so you need to be careful.
Creatures: Creatures have the most room to work with, with many different balanceable variables, but they are generally fragile. Many creatures may become too complicated, however, if you take it overboard in mechanics.
Step 3 - Theme
At this point I have decided that I am creating a Creature that has the passive ability Reflective.
Now I must work on the theme of the card.
There are several sub-parts, especially in creatures.
A) Name
The name dictates what the rest of the creature will be based upon, so be careful. I chose "Mirror Golem" because that was what came to mind when I thought of a creature that reflects. If I chose a different name it would be a different card. In spells and permanents the name is less important, but make sure to pick something thematically appropriate.
B) Element
Now that I have a Mirror Golem, what element does it belong to? I decided that I somehow attribute glass and mirrors to Aether, so that's what I picked. If I picked an element like Death however, it would have been thematically out of place. If I wanted to make a Reflective Death creature, I would have to consider changing the name.
C) Art
Even if it's a filler, at least make something that looks vaguely like it. This makes the card look more complete and thought out, and enhances the theme.
D) Thematic Support Abilities
More for creatures, you may want to add abilities secondary to your original concept. An easy pick is Airborne.
Since Reflective is passive, I chose to give it a growth ability to match Lava Golem. Thematically, A Mirror creature would become stronger with
, so I chose that as the cost.
Step 4 - Balancing
The next thing you do is to balance your card to the current metagame. Basically you change a variety of factors to reach a target cost.
A) Cost/Play Speed
Cost of 0-2 is "Splashable" or "Quick".
They can be easily powered off mark or off Novas, and are generally weak to compensate for it. If you don't splash them,m then generally they are designed to hit the field quickly or whenever they need to, since you always have the quanta for them.
Cost of 3-5 is "Standard"
They can no longer be easily splashed into decks, but fit easily into any duos that already exist with them for some extra damage or support. This is my favourite range, and this is where Mirror Golem fits.
Cost of 6+ is "Dedicated" or "Late-Game"
At this point you aren't just adding it for support, there's a specific function the card plays. At this range you need to either dedicate a large amount of your deck into getting this out quickly, or you add only a few copies for late-game quanta usage.
B) Fragility
How easily should it die?
Usually this means the HP amount.
The less HP it has, the more things can kill it in one hit. Since Mirror Golem has reflective already, I chose to make it fragile with only 2 HP. Likewise, I made it's growth ability not give HP so that it doesn't become excessively difficult to get rid of later.
C) Power Niche
This should be adjusted to match the Cost.
1-3 ATK: Weak. Usually there's an ability that makes these worth playing. Usually they are not there for damage but rather have an ability that needs to hit the field as soon as possible. 3 attack however has an Adrenaline niche so it makes them playable in Life duos.
4-7 ATK: Midrange. These are the general damage cards that fit into decks the easiest. Most popular for rushes.
8+ ATK: Heavy hitters. Sacrificing play speed for damage potential, these are the kinds of cards the bust past Stalls.
You generally want to put your creature in a niche that isn't yet filled.
D) Ability Strength/Cost.
I could've made Ashine cost 3
and give like +5/+0. But that kind of ability is pointlessly powerful.
Make sure your ability is well balanced in that sense.
Step 5 - Community Feedback
Post your idea! get some feedback on how it could be improved. Even if you think it's perfect there's always something to work on.