The bermuda should not be heavily fertilized after August 31, since nitrogen will favor more growth and less food storage. Excess fertilization of bermuda in late August can make the bermuda more competetive than it needs to be with the overseed. The damage usually shows up in the spring as poor transition to bermuda in May or so. Severe dethatching should be done in the summer and not at overseeding. This weakens the bermuda for winter survival and spring transition.
Homeowners are better off to lightly verticut in at least two directions.
After that, the lawn can be scalped. The scalped turf is more open to receive the seed during overseeding. After the scalp, pick up the clippings with a rotary mower and bag them, or cut them in place by mowing the repeatedly. Now it's time for seeding. Perennial ryegrasses are finer in leaf texture, can be mowed either with a rotary or reel mower, and are darker green in color than annual ryegrass.