Open up both ends of the 3/8" line, pulling a little core out of each end. It helps if you loosely tape up the jacket to stop it unravelling. Find the center of the 6' length of Marlow, and sew it end-to-end to the core in the 3/8" line. Sewing the Marlow to the core can be made easier by whipping the core and then sewing the Marlow to the core with the threads going behind the whipping, such that it anchors them rather than having them simply pull through the core.
Using the core at the other end of the 3/8" line, pull through some core, such that about 8" of the doubled up Marlow are pulled inside the jacket of the 3/8" line. That is 16" total or 8" of the doubled. Sew the end of the jacket to both the strands of thin Marlow. Sew through the line tying the jacket to the two Marlow strands up to where the Marlow is sewn to the core, and a little passed that. All sewing should be done with strong waxed whipping twine.
Stretch the mainsheet jacket back over the core, and cut off the excess core at the other end of the 3/8" line. Sew up that end as well.
Sew small loops in the bitter ends of the thin Marlow or spectra. Shackle those loops to eye straps on the rail of the boat, near the transom. The length of the two pieces of Marlow is rather critical. Not only should the distance from where the two strands of Marlow are sewn to the 3/8" line to the bearing surface of the loop be the same (so the bridle is centered), but the length should be such that you are just starting to pull the Marlow into the boom end block at 25' 8" rake and a light air vang setting. If the bridle is too low, you will be pulling down on the boom rather than to the corner of the transom, and if it is too high, you risk getting a loop of thin bridle line around the boom end block. You can check that the bridle is centered by stretching it up and lining it up with the pintles/gudgeons on the transom. You can also make the bridle tail lines a little too long, tie them on, and get the correct length and centering.. then sew the loops.
Lead the other end of the mainsheet through the block on the outboard end of the boom, through a couple of hold ups (I use webbing riveted to the boom), and then through the ratchet block in the center of the boom, then through the block on the swivel jammer and through the eye strap on top of the cleat. Take the bitter end of this mainsheet and tie it to the boom fitting which the ratchet block is hanging on. This end becomes your pumping and gybing line.
You still need about 27 feet of 3/8" line. Buy the cheapest line with a jacket you like, as you will be throwing away the core. I like Sampson Trophy Braid for this application. Buy 33 feet of Spectra core (no jacket) line. The thickness should be a little over 1/8", about the same as the core in the 3/8" line. Cut 3' off one end of the Spectra and then sew that to the main piece of Spectra 3 feet in from one end such that one end of the 30' portion lines up with one end of the 3' portion.
Temporarily sew the other end of the 30' Spectra to the core of the 3/8" line (you have to open both ends of the 3/8" line as for the other mainsheet approach). Tie the non sewn end of the 3/8" line's core to something immovable, and then drag the jacket off the old core, onto the Spectra line. This can take some time. Keep moving the jacket over until you the point where you sewed the two Spectra lines together is inside the jacket by about 8".
Sew up the jacket over the two tails. Cut off the core or Spectra where it emerges from the other end of the 3/8" line and sew that end up. Sew the loops in the thin tails. Install as you would the other variant.
This second approach is very strong, light and low stretch. When the jacket on the mainsheet wears badly at the cleat and ratchet block, simply undo the sewing at either end of the 3/8" line, strip off the old jacket, and slide a new one on.
My experience is that a slightly fuzzy jacket like the Aracom 80 or Trophy Braid is easier to hold onto. The Spectra line seems to be almost as durable as the thin Marlow. I have not had wear problems where it is shackled to the eye straps on the rail.
What I found with a couple of variations of this is that sewing unprotected by a jacket rapidly broke down, and that a very thin mainsheet would occasionally tangle on the boom end block, making it difficult to let the main out! None of these mainsheets would last without continual maintenance.
The problem with this is that a thin line hanging next to the ratchet block on the boom will sooner or later be sucked into the ratchet block as you trim or ease the main, and will jam the mainsheet in the ratchet block. If this sounds a little farfetched, ask me to tell you about the race in the UK Nationals where we sucked our spinnaker sheets through the ratchet block.... several times!!
You might be able to make this work by deadending the pumping line further away from the ratchet block.
This actually works quite well, though the resulting mainsheet is oval in cross section. It takes quite a bit longer to slide the jacket on, as you have to work the twists in the two thin lines out as you pull the jacket over them.
Check the lengths of the two bridle lines. They should join exactly on the centerline of the boat if you pull them taught. When you pull the mainsheet tight, you want the boom centered, not off centered!
You could eliminate the loops in the thin Marlow, the shackles and the eye straps by simply drilling a hole in each rail, slipping the end of the Marlow line through the hole, and tying a knot on the other side of the hole. Watch for wear on the bridle line on the edge of the hole if you do this.
If I remember, I will measure the length of the 3/8" line, and the length from the join to the bearing surface on the Marlow lines, on one of my mainsheets, and add those numbers to this article.