Hi,
Thanks for asking about pruning properly before going at it with a hedge clipper.  I'm sending you a few good articles to check out.  In the first one, read the section (appropriately) called "Pruning".  If you do what it says and remove the older stems and you still want your plant to be smaller, check out the link for "Pruning Large Overgrown Shrubs".
https://yardandgarden.extension.iastate.edu/how-to/growing-lilacs-home-garden
The link will go to
https://yardandgarden.extension.iastate.edu/how-to/pruning-large-overgrown-shrubs
The more severe pruning is called renovation or renewal pruning.  It is severe and you will lose blooms for at least one year but in the long run, you should end up with a nicely shaped "new" lilac. 
Let me know if you have any more questions. 
Barb



https://www.canr.msu.edu/news/pruning_spring_flowering_shrubs

You're welcome.  
Good luck.