If your lawn has not been fed in the last 4 - 6 months then you should feed it 3 or 4 days before over seeding. For best results spike the lawn first with a fork, down to the depth of the fork tines, every 300mm (1 foot). This will help get the lawn food into the soil where it can be absorbed by the lawns roots. If you have fed your lawn more recently then spike the lawn with a fork and over seed. You should plan to then feed the lawn within the next 4 - 8 weeks.