Help for Sum of Square Reconstruction in Parallel Imaging (MRI)

I am trying to have a reconstructed image by utilizing the parallel imaging technique. The algorithm I am interested in is PILS.

I am using the Sheep-Logan phantom available in the MATLAB.

I have created 4 different coil sensitivities, as Gaussian. Center of each coil sensitivity represents different points on the image in phase-encoding direction.

After I reconstruct corresponding images for each coil from decreased number of lines in the k-space data, I can not reconstructed the final/combined image including all 4 coil images. At this point, I need to know about the Sum of Squares reconstruction method. But I am suffering from this information.

Could you please explain me how to obtain the combined image with these 4 individual coil images using SoS reconstruction?