Yes, you can do it now. I'd remove the leaves first. While they would
make a nice humus, having them mixed in will make it impossible to rake
and level. If you can get some compost material, that would be ideal
to mix in. It's likely the soil will need lime, so I'd get it tested
either by an agricultural service or with one of the self test kits you
can buy. Add in the right amount of lime before tilling, use a starter
fertilizer, and keep it constantly damp until it germinates, then
gradually back off watering.

I'd make sure to use a quality grass seed. Check out seelland.com,
they have a good selection and info.