Let us start with the when - Lilacs should be pruned just after the flowers have finished, as they set bud immediately after they are finished flowering. If you prune before or after this period, you will have less blooms the following season. That being said, if your hedges are overgrown, it may be easier to start before they leaf out in the spring so you can really see what you are dealing with.
As with any pruning, the general rule is to take out no more than 1/3 of the shrub each year - so to rejuvenate your hedge - you will be looking at a three year job. You can follow up with yearly maintenance pruning after that.
Start by taking out any dead or diseased branches first. (I don't personally consider "dead" to count towards the 1/3 of the plant.) Move on to taking the oldest and thickest branches, especially those larger than 2" in diameter (this will likely cut down on the height of your hedge.) Cut or saw them off as close to the ground as you can get. Move on to suckers that come from the base of the plant that are the same as or smaller than a pencil and finally any rubbing or crossing branches until you have removed approximately 1/3 of the plant. To encourage the shrub to "fill out", when making a cut, prune the remaining new stems to an outward facing bud.
Repeat this process for the next 2 years. With regular pruning and removal of the old stems you should be able to keep your hedge at a manageable height. Do not just take back the tops of the too-tall stems as this will not do anything to rejuvenate the Lilac and will leave it in an unnatural shape. It is better to fully remove the over-grown stem and allow for new growth to fill in. After the 3 years, the majority of the plant should be new growth - and you can begin a maintenance pruning schedule right after it has finished it's blooming cycle (Deadhead (if you wish), then follow with dead/diseased, branches larger than 2", suckers and crossing branches - keeping in mind that you will not always need to take out the full 1/3 of the plant. Use your best judgement.
Healthy, and happy plants will produce more flowers and live longer.