They are indicators of keeping (+) or reversing (-) whatever sign the number originally had. Note that the (+) or (-) signs in the checkerboard diagram do not suggest that the final term should be positive or negative.
Continue on with the rest of the matrix in this fashion. The third element keeps its original sign. When assigning signs, the first element of the first row keeps its original sign.
You must then reverse the sign of alternating terms of this new matrix, following the “checkerboard” pattern shown. Thus, the determinant that you calculated from item (1,1) of the original matrix goes in position (1,1). Place the results of the previous step into a new matrix of cofactors by aligning each minor matrix determinant with the corresponding position in the original matrix.
