Adding fertilizer to your lawn at the right time with the right mixture can keep your lawn growing, healthy, and looking great. It is important to know based on your climate when is the best time to fertilize, although spring and fall are good general rules of thumb. Make sure not to fertilize too much, as this can cause your lawn to burn creating a really poor appearance and a possibility of having to replace it.
The three big things to fertilize your lawn are getting the right fertilizer for your situation, getting a rolling or hand spun spreader, and the timing of when to do this. Fill the spreader with fertilizer, open the valve and begin briskly pacing up and down the lawn to ensure an even spreading.
Here is a great video from The Home Depot on fertilizing your lawn.