How to Find the Coordinate Transformation that Diagonalizes a Quadratic Form?

[QuantumDefect]

Hello, I was wondering how one would go about finding the coordinate transformation that diagonalizes:
2(x1)^2+2(x2)^2+(x3)^2+2(x1)(x3)+2(x2)(x3)

Thanks a bunch.

 

[QuantumDefect]

maybe just a tip on how to organize the above equation in the form of a matrix.