Choose a win condition
Add a secondary win condition or support plan
Include one or two reliable tank killers
Include one or two splash damage cards
Include one small spell
Include one big spell
Include one building or defensive control card
Include cheap cycle cards
Keep average elixir balanced
Make sure air defense is included
Make sure ground defense is included
Add cards that work well together
Avoid too many expensive cards
Avoid overlapping roles
Test the deck in battles
Replace weak cards after testing
Adjust the deck for your playstyle
Balance offense and defense
Match card levels as much as possible
Build around a clear strategy