Scarification of your lawn is an essential process to ensure a healthy lawn. It should be done at least once a year - either in late spring or autumn. Do not do it too early in the spring as the grass is just starting to grow again after winter and you may remove a lot of new shoots.
The scarification process is a more intense removal of thatch, moss and dead grass than using a lawn rake. Scarifiers typically have circular blades that cut through the grass and into the earth, so also help aerate the soil which is another important process to ensure healthy lawn grass. Lawn rakes typically have spring tines that rake at the grass and are effective in removing moss and some thatch but do not cut into the soil.
You should conduct scarification as part of an overall process to get your lawn onto a good footing for the summer. Give the lawn a cut a few days before scarification and then try and time the scarifying a few days before rain.
When scarifying you will need to make two passes over the lawn at right angle to each other so that you can remove as much thatch as possible. Your lawn may look quite bad after this rigorous process but it is a well established method of dealing with problems such as poor growing areas, bare patches and over growth of one type of grass over another.
After scarifying, within a day out down a feed onto the lawn to help the grass re-establish itself and also take the opportunity to reseed the more bare areas and those where you do not have a good mix of grasses. As the soil has been aerated as well the seed should establish well. Finally give the lawn a roll after feed and reseed to help these settle into the soil. If no rain is forecast in the next day then give the lawn a water.
The lawn will look a bit bare and rough for a few weeks but it should re-establish well.
In terms of which scarifiers to use - hand ones are available but it is back breaking work. These will do for the smallest of lawns. For mid to larger size lawns electric or petrol driven ones are available. Certainly for a large lawn the freedom of a petrol/gas driven scarifier is a lot easier than an electric one trailing a power cord. Also petrol ones will be more powerful. A good example is the At-co range which come in electric or petrol at decent prices and also have combination of scarifier and aeration cylinders and also a rake a seperate cylinder to install.
It is recommended that raking also be done at least a couple of times a year, but this should follow a weed and feed where this will focus on killing dead moss and a rake can then be used a week or so later to remove this. Again moss can be prevalent after winter so a good time is to do an initial weed and feed in early spring and then use the lawn rake to de-moss.