Numerical simulations of the manufacturing of waveguides with grating patterns on a nonlinear optical (NLO) polymer film using photothermal poling were carried out. The mathematical model was constructed, in which spatial and temporal variations of molecular dipoles during heating with a focused laser beam and electrical poling were considered. The effects of operational factors, i.e., laser power, beam diameter, moving speed of a sample, and electric field strength, on the grating configuration were investigated. As a result, the following guidelines for producing a waveguide with high performance were obtained. The laser beam wavelength should be selected so as to heat uniformly throughout the thickness of the polymer film, the laser power should be as high as possible without causing large deformation in the film, the laser beam diameter should be smaller than half period of the grating, the moving speed of the stage should be slow, and the electric field strength should be as high as possible, if dielectric breakdown is to be avoided.