1. Straighten pipes and tighten nuts
The washers that seal pipe joints won’t hold water unless one section runs straight into the other. The “ground” joint on the trap has no washer, but it too will leak if it’s misaligned (see p. 51). Eyeball the leaking joint to check its alignment. If it’s crooked, simply loosen the nut, straighten the pipe and retighten. Since the whole assembly is interconnected, you might misalign one joint while straightening another. Don’t be surprised if you end up loosening and tightening several joints to straighten just one.
If a joint is aligned but leaks anyway, tighten the slip nut. Use two slip-joint pliers on metal pipes: one to hold the pipe, the other to tighten the nut. If you have old metal pipe, you might find that it has worn thin and collapses when you put a pair of pliers on it. With plastic pipe, hand-tighten first. If that doesn’t stop the leak, use pliers. But be gentle; plastic threads are easy to strip.