In order to provide a practical framework for the calculation of diabatic (technically quasidiabatic) states, we generalize the diabatization procedures of Atchity and Ruedenberg to include more general types of crossings and avoided crossings of potential energy surfaces. The resulting diabatization procedure involves two steps: (i) the construction of diabatic orbitals and (ii) the construction of many-electron diabatic state functions in terms of the diabatic orbitals. The procedure for step (i) is more general than the previously proposed occupation number and natural orbital method, and the procedure for step (ii) remains valid even for chemical reactions that require multiple diabatic prototypes. The method is illustrated by applications to LiH, ozone, H-2 dimer, and the reaction Li(S-2,P-2)+HF-->LiF+H.