O.K. so you wake up one fine morning and discover - horror of horrors - that there are large bumps and hollows in your lawn. Or, perhaps you have just moved house and need to deal with a new garden.
Well first off, don't panic! Second off, forget your garden roller as a remedy! Using a garden roller simply enhances the bumps and hollows and compacts everything into a hard mass.
If you have "large hollows" in your Lawn, the only way to get rid of them is to strip off the turf and take out some soil before replacing the turf in its original position.
However, often times, it is simply an uneven lawn that is the problem. If this is the case then the easy answer is to bring the hollows up to the level of the bumps, thus creating a flat surface.
To define this in a more clear manner, if the hollows are about 2" deep then the lawn can be treated easily by filling the hollows up.
First thing to do is find a straight edge, perhaps a plank will do. Place the plank over the hollow to be treated and use it to spread a top dressing mix into the lawn to a rough level by "screeding" the edge back and forward.
Once the surface is level, then compact the soil by treading in with your heels. The final result should be only sufficiently firm as to not leave any fooprint.
Then rake the soil to its final level with as wide a rake as you can find.
Finally, spread a mix of grass seed over the treated area at about 1 good handful for each square yard, making sure of a wide, even spread across the area.
Rake this in lightly.
I suggest you protect your lawn against the ravages of birds, possibly with some form of net protection. Water the area and await results for about 2 weeks. By then the grass should have started to grow and you will have a lawn that is level and flat.