Homebrew is what makes the game yours. But unbalanced homebrew compounds over levels.
The 3-Test System
Test 1: Comparison - Find the closest official equivalent. Stronger or weaker? Test 2: Edge Case - What about multiclassing? Other magic items? High-level play? Test 3: Table Test - Run it for 2-3 sessions as a beta.
Balancing by Category
Subclasses: Match or slightly below the strongest official subclass. Monsters: Use CR as starting point, adjust for action economy. Magic Items: Follow rarity guidelines for mechanics. Add narrative freely. Spells: Compare to same-level spells strictly.
The Golden Rule
Start weaker, buff later. Players love getting stronger. They hate nerfs.