How do i trim lilac bushes, and when?

This article has loads of information on when and how to prune lilac baushes. https://www.gardeningknowhow.com/ornamental/shrubs/lilac/pruning-lilac-bushes-when-to-trim-lilac-bushes.htm

The best time to trim lilac bushes is right after they bloom in the spring and they all turn brown, Because then you can prune all you want so then it has the rest of the year to set buds for next spring flowering. You can prune now but you will not have a lot of lilacs in the spring. And keep the suckers growing around the bottom of the trunk if you want to control the size of bush? You can trim off the flowers anytime when in bloom, but branches and size you need to do as said above.

cut after it blooms in spring

What I meant to say is And keep the suckers cut from growing around the in the soil by the bottom of the trunk if you want to control the size of bush? Thanks for the reply earlier.

Also anything you cut off the main trunk always cut near as close as you can at a angle so the rain water runs off and does not keep soaking in the trunk and cause a disease but you can trim high top branches lower which it will then start growing branches down lower so you can get lilacs in areas you can get to.