Measure the length and width of the yard with a tape measure and multiply the two resulting numbers together to get your yard's total square footage. Take this number with you and purchase enough bags of topsoil or fill dirt to accommodate for your yard's space. Mow your lawn to get the grass level down as low as possible to expose the lumps as best you can.
Open the bags and toss the soil or dirt onto the lawn to fill the crevices between the lumps until the soil is level with the tops of the bumps. Once all is filled to satisfaction, stamp the soil down with a soil tamper and reseed the new soil with grass seed. Leave your lawn undisturbed while the seeding takes place.
Cut and remove the sod from the lumpiest parts of the lawn and use a tiller to till the bumps down until they're level with soil line. Replace the sod, tamp it down and reseed if necessary. You'll need to leave the yard undisturbed to allow the sod to take.