Personally, the first thing I would do would be to take out the plastic liner and then start filling it with soil.
Fill the remaining hole with spare soil until it stands proud of the adjacent garden to allow for settlement. Repeat until it is pretty much level with the adjacent area, then seed.
You don't say how deep the pond was, but you might want to consider throwing rocks, stones or rubble into the hole before filling with soil to allow for decent drainage.