Pruning a lilac bush what is the best way

Trees can get very tall it depends on the type, didnt you keep any instructions? May even need to prune the top leaves so it spreads out.

You can cut them right to the ground and they will come back after a couple of years as long as the roots are left in. The best thing is to cut out the twisted and knarly looking branches, and all the dead flowers. This is the best time of year to hard prune them. You might find some useful tips on the RHS website.

Prune in the spring time. Cut diagonally away from the bud. You can take up to 1/3 of the height.

not with a chainsaw !!!!!
http://homeguides.sfgate.com/prune-granny-smith-green-apple-tree-56671.html
http://www.pickmarin.org/prune-an-apple-tree.html
https://www.youtube.com/watch?v=Qar8TmsobPk

Pruning Lilac Bushes When To Trim Lilac Bushes
This should help.

You have gumosis. Trim off infected area with a saw blade sprayed with a 2% bleach solution. Look up proper pruning practices prior to pruning.

definately in the fall