It's best to plant in September, first doing a shallow tilling or adding several inches of compost. The compost must be well rotted and fine to hold moisture if you plant the fern directly in it. Tree roots and hard soil will slow, if not prevent, establishment. So help the fern out by breaking up a few surface roots. And if they get a few hours of dappled sunlight, you're likely to be impressed.
Growth is much slower in total shade.
That's all the plant will need until spring. In March, before new fronds emerge, repeat the fertilizer application, watering it in again. Try to keep the fertilizer away from the plant crown. May. Watering cannot be overstressed. You can't allow the newly established autumn fern to ever dry out.