Undergrad Relation between tensor decomposition and helicity amplitude

[CAF123]

It is common to write e.g photon two point function in terms of manifest transverse and longitudinal form factors with lorentz structure factored out, e.g $$\Pi^{\mu \nu} = (g^{\mu \nu} - q^{\mu} q^{\nu}/q^2)T_T + q^{\mu} q^{\nu}T_L,$$ where mu and nu are polarisation indices.

How do I relate such expressions to helicity amplitudes which are usually given in terms of e.g ++, -- for transverse degrees of freedom or 00 for longitudinal?

If I take two transverse photons, then mu and nu are both +, but then what is $g^{++}$? Looks like light cone components but I am not sure if this is the correct notation.

 

[CAF123]

Perhaps put another way, how to go from helicity summed expressions to the individual helicity components?