Constructional stretch occurs as a byproduct of the splicing method. When you open the weave to bury the tail, you also cause the line to shrink in length. When tension is applied to the line, this looseness in the weave will work itself out in the form of stretch.
It may seem that a new stay is stretching like crazy, but this is all due to the splicing process. Removing this stretch before installing the stay will greatly decrease the amount of time it takes to setup new synthetic standing rigging.
To begin, you need to stretch out all your newly spliced stays and line them up end to end. You will tie the end of the last stay to a strong anchor point, the other end to a strong pulling device, all the stays in between will be tied to each other.
I like to use a massive tree with a strong root system as my anchor and a heavy truck as my pulling device. With the stays tied between the truck and the tree, I can create a lot of tension on the stays to remove most of the constructional stretch.
Once everything is setup, the process of removing the stretch is very straight forward, simply drive away from the tree and have the synthetic rigging stop you. The truck I use weighs around 7,000 pounds, so stopping it requires some serious force. This strong yank will quickly remove any stretch that might still be present in the splices.
The process of removing the constructional stretch is very straight forward, but knowing when you are done is a little less obvious. There are a few ways to check and see if the constructional stretch has been removed, some are more obvious than others.
The first way is to milk the splice from the eye splice into the stay. If there is constructional stretch present, the outer weave will begin to bulge. Working these loose strands into the stay will greatly remove constructional stretch. If you can't get the outer weave to bulge, most of the constructional stretch has been removed.
Another way to remove the stretch is to push up and down on the stay while it is pulled tight by the truck. I push the stay down with my foot in the middle of the length where my force will induce the most tension. If there is constructional stretch present, the tight stay will become slack when I release my foot. If it becomes slack, move the truck forward to tighten the stay again and repeat the process.
Both of these methods involve close inspection of the stay, and a lot of walking. Walking from the truck to the splices and back to the truck over and over takes a lot of time. Instead of all this walking, an easier method is to simply drive the truck forwards and backwards repeatedly. The sharp yanks induced by the truck will remove the stretch in a very short amount of time. After a few yanks, you can feel free to do the first two steps and evaluate if any stretch is remaining in the stay.
The easiest way to evaluate if any constructional stretch is present is to look at the thimbles. A new thimble that has never been loaded will have an opening near the throat. After a thimble has been loaded with a lot of tension, the throat will close and the ends of the thimble will touch.
After a few pulls with a truck, you will see the thimbles starting to close. Simply repeat the process until all the thimbles are closed, then evaluate the stays by milking and pushing down in the middle. If all the thimbles are closed and there is no remaining stretch, the stays will remain tight after testing them with your foot (pushing down in the middle of the stay and releasing to see if the line goes slack). If there is any stretch in them, simply do a few more pulls with the truck and re-evaluate the stays.