slide the white covers up that are at the bottom of the shrouds. You should find the shrouds are attached to fittings that have holes so you can move the shroud down to tighten the rig. You should look for a P-16 Assembly Manual on line and read it.
Are you saying there is no boom with the boat?
These are called "chain plates" and should have a series of holes in them to adjust the shroud length where it connects to the hull. These are normally left very loose so you can step the mast and pin the forestay without any resistance. Once stepped, someone usually hangs off a trapeze line to preload the rig and then a second person pins the shrouds as low on the chainplate as possible. Adjust them side to side to get them mostly even while tight. We tighten them so the rig doesn't flop around in light air (causing a slight loss of performance and adding premature wear on things)...but when sailing in any kind of breeze, the leeward shroud will always be loose. Tension isn't super critical.
