I am going to quote my source directly for this:
"For the very oldest and most overgrown forsythia, pruning should be brutal, cropping the entire shrub to about 4 inches from the ground."
If the bush is truly out of control, that is your method. Forsythias that are well established are extremely resilient, so it won't kill it. Ideal timing for something this drastic is likely during dormancy, but avoiding the worst of winter. So either just after it loses it's leaves or just before it starts to bloom again.
Over the course of the next two years it will re-establish itself as a small shrub, then it's up to your pruning regiment to keep it to a manageable size. Normal maintenance pruning is best done just after the blossoms fade in the spring, before the forsythia has a chance to set new buds, which it does shortly after blooming. Once it's getting back up to your desired size, start pruning out the oldest sets of branches, this way the plant grows new growth every year, but doesn't expand and try to consume your friendly local everything.