Least Squares Re Synthesis of Short Time Fourier Transform (STFT)
The least-squares re-synthesis procedure is very similar to the overlap-add (OLA) procedure.

Let w_n be the discrete window vector and y_n(:,k) be the time domain vector computed using the IFFT on a column k of the 2D matrix. Both w_n and y_n(:,k) are the same length.

Then, using Matlab syntax, we compute the point-by-point multiplication with the window:

w_n .* y_n(:,k)

As mentioned in the comments above, the y_n is trimmed to be the same length as the window w_t.  The w_n .* y_n(:,k) is then overlap-added in the same fashion as shown in the code associated with my previous post on the inverse STFT here.  The overlap-added sequence is the numerator of the expression shown in the original question above. 

The same overlap-add operation is applied to the squared window w_t.^2.  This is the denominator of the expression shown in the original question above.

Then, the final re-synthesized output is simply the point-by-point division of the numerator by the denominator.  In Matlab syntax,

numerator ./ denominator