Translating Schenkgott might be gold to start.
In general I think I follow the following steps:
Step 1: Pick a mage.
If you are not familiar with the options, look up the mages and their respective tactics and strengths/weaknesses.
Step 2: Pick a theme/strategy/card around which your deck is designed.
It is very important that you choose a card or a theme which your mage will use as the main goal to kill the opponent. After you have decided what you want, you should try to include all cards necessary to support this strategy. Question yourself what is necessary to keep your strategy going. The trick here is to only add cards needed for your strategy. If you add every card that might be helpful than you will have way too many cards.
Examples:
Warlock, Sectarus: Include a battle forge, enough curses, something to increase the strength of your mage.
Beastmaster, Animal Kinship: You need enough cheap creatures to keep the kinship effects going. Probably a lair to summon them.
Warlord, Talos: Include many outposts so you are sure to summon talos. Try to get a good mix between level 1 and level 2 creatures. In the beginning you are short in mana so the small ones are preferable while after a couple of rounds it is probably better to summon more durable creatures. Include the dwarf panzerguard to guard outposts.
Step 3: Cover your decks weaknesses.
This is probably the hardest step during spellbook creation since it is not always easy to predict possible counters against you. With practice you will become better at this. After you have played with your deck you can always adjust a few cards to improve this.
Some (but far from all) common weaknesses:
Movement/positioning: Especially Solo/buddy builds need enough positioning and movement cards since every hit with your strong creature counts. Cards like force push/teleport/charge/Mongoose Agility/cheetah speed and even walls can help you with this.
Flying: If you don't have many flying creatures and don't plan to use many attack spells it might be wise to either include one flying creature, eagle wings, maim wings or gravikor. Especially druids and necromancers have problems against flyers.
Rushes: If you have a slow startup, make sure you have at least some (cheap) means to protect you at the start of a game. Common used cards are Defend, Intercepting creatures, walls, Brace yourself, and many more.
Mana drainage: Especially swarms have problems with mana and they can be drained even more easily with cards like [mwcard=MW1J21]Suppression Orb[/mwcard] and [mwcard=MW1J16]Mordok's Obelisk[/mwcard]. Either add more economy cards, a [mwcard=FWJ05]Mana Prism[/mwcard] or make sure to have some means to take out conjurations like that. (1 or 2 rounds)
Step 4: Include a backup strategy
Imagine that your strategy will not work for any reason at all. Think of a secondary tactic you can use if all else fails. Preferably this strategy uses many of the same cards as your main strategy but in a different way. This is of course not always possible.
Examples:
Spawnpoint is destroyed, no swarm possible: Include one or two very big creatures and change your strategy more into a buddy build.
Your deck is based on tainted or poison and you face a necromancer.
You want to use fire but the opponent is using too much fire protection for you to handle.
Step 5: Include some staple cards.
Not all of these cards are per sé necessary, but make sure you checked anyway.
- Dispel/disperse/etc... A must in every deck.
- Dissolve/Crumble/etc...
- Some Positioning/movement cards
- Walls
- Some fire protection: Without it, the adramelech can potentially destroy you.
- Healing: not every deck needs healing, but just remember bleed. Without healing this can be devistating
- Purify
- Armor: Every mage should add some degree of armor. Defences are very nice, but wall of thornes is unforgiving.
- Means against armor: If the opponent has 4 armor and you are not using spells like "Ghoul rot" your damage output can be very low. Keep in mind that there are cards like defences, Voltaric shield, healing, regeneration, ... that can make this problem even larger. Common spells: Dissolve, Disperse, Acid ball, Rust, or a creature that causes corrodes.
- Some attack spells: Every mage needs a few attack spells. Not every strategy uses a lot of them but reserve at least 8 spellbookpoints for attack spells.
Optional Step 6: Add a card you really want to try out but probably won't need
Since it's still a game designed to have fun, I suggest to have at least a few cards you will almost never use but are just very cool. Granted, it will not improve the strength of your deck in most situations. But if you ever do have the chance to use it, it can give you a huge boost and it is just super fun!
