How does one make a balanced variant?

Out of curiosity, how does one make a board that is balanced for both sides (ex: such as one to make white going first not unfair for black in some way?

Asking in regards to the custom pieces (that I may or may not be bad at using well)

Testing. A lot of testing. Every early attack white can do, black should be able to defend against.

Thats a problem in Cloister

"Every early attack can be defended" is a starting point. "Every early attack has a counter-attack" also works (like in Cloister), but is harder to perfect (unless, it is giveaway, where counter-attacks are basically the only way to defend).

oh

Your goal is to bring this principle to fact:

Chess played perfectly is a DRAW.

Following that principle is only ok if the variant is complex enough that it is humanly impossible to play perfectly every single move (e.g. players would realistically play a bad move somewhere). Following this principle on less complex variants just makes the game a forced draw. This can be solved, however, by adding lives or a hill.

Geniuse

Generally speaking, a symmetrical board layout tends to be more balanced. This is not always the case, but you might notice that many of the listed variants on the server are symmetrical in some way, which is a testament of their relatively balanced nature.