We want to write a generic Lorentz matrix
as the product RB,
where R is a rotation and B a boost. The generic boost matrix is
(Jackson, eq. (11.98)):

where
are the Cartesian components of the boost velocity
.
On the other hand, R is of the form

where
is a three-dimensional rotation matrix. The problem is to
determine B and R, given
We can keep the indices consistently placed (up or down) by rewriting B in the form

where
are covariant components. We find

hence
and

or explicitly
