The setup to tune synthetic rigging with deadeyes is very simple, all you need is a turning block, and some leads that can be run to a winch. This allows you to use achieve the necessary tension in the stays in a controlled and repeatable manner.
To begin, the turning block is tied to the toe rail where a fair lead is in plane with the deadeye. This will ensure that the force on the lashings is vertical with no horizontal component.
Once the turning block is positioned properly, untie the tails and lead them through the turning block, then over to the tensioning line. The tensioning line (9mm VPC) is led to the winches in the cockpit. I use the secondary winch for the anchor and the primary winch to drive the tension. To connect the tails to the VPC tensioning line, I use a sheet bend with a slip; this knot will hold securely as incredible loads are applied to the system, but is also possible to untie after the procedure is completed.
One important trick when tensioning the stay is to set everything up as seen in the photo above, then apply tension to the whole system before untying the shroud frapping knot. This will make sure the tensions are similar between the tails and lashings once the shroud frapping knot is untied.
Once the shroud frapping knot is untied, tension can be added to the stay via the cockpit winches until the appropriate amount of tension is present in the stay. I perform this adjustment on both sides of the boat at the same time to ensure that the mast stays in column and that the stays are of similar tension. This does require double the setup materials, but the end result is very even tensioning of the standing rigging.
For more details into how to tension your synthetic standing rigging with deadeyes, please read this post.