Answer: The best thing to do would be to work with your local county extension to run some basic soil tests and see what amendments would be best to use and in what quantities.
You want to minimize tilling as it brings weed seeds to the surface where they will germinate and can also be deleterious to the soil structure. So while you might want to till in certain amendments now in preparation for spring, you do not want to till more times than necessary.
Fertilizer is typically spread on the soil surface; it will leach through the soil so it is wasteful to apply that now without grass in place -- it will keep in the bag until spring.
You may need to adapt the pH (it is probably acidic and needs to be limed to raise the pH to be more suitable for lawn grass; lime is slow acting and since you are just starting the lawn, tilling it in is a good way to spread it through the soil in a hurry) which can certainly be done in the fall and may still require additional adjustment in the spring.
Generally speaking, clay soils can be improved by the addition of organic matter such as good quality compost, rotted down leaves, well aged stable/bedding, and so on. You may also need to add some coarse sand.
Mulch can supply organic matter to the soil which is often a good amendment in clay soils, and you could till it in now along with the lime -- if the soil tests show you need lime. That will allow the mulch some time to begin breaking down and feeding the soil before you seed again in the spring. This would depend somewhat however on what material you are using as mulch. Bales of straw for instance can be stored dry for use in the spring when you seed. You would not want to use a bark or wood mulch with grass seed, so that would be better either worked into the soil now or used elsewhere such as around shrubs.
I know you are eager to get started and are looking at bare soil, it is possible that some of your earlier seeding will still germinate this fall. (Germination is slower the colder the soil.) Once it starts to germinate, you do not want to disturb the soil -- the disturbance will kill the germinating seeds and newly sprouted grass.
The county extension should be able to help you determine what would be best to do at this point based on a more detailed understanding of the soil conditions you currently have. They may also have suggestions as to how best ot protect the soil surface this winter if you do not have grass coverage. Good luck with your lawn!
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