First, in order to be sure we're all on the same page, it might be a good idea to straighten out a little vocabulary. The heavy line leading from the top of the fore lower mast to the deck ahead of it is the forestay. The one leading from the top of the main lower mast is the mainstay. The mizzenstay leads from the top of the mizzen lower mast to a fitting on the aft side of the main lower mast, near the deck.
It's been a long time since I've built the Revell kit, and I don't have one in front of me, so I'm relying on the George Campbell plans of the real ship. They indicate that the forestay "runs in two parts" - i.e., it is in effect doubled throughout its length. It's set up to two pairs of hearts and lanyards, which are secured to eyebolts in the aft sides of the knightheads - the big, near-vertical posts on either side of the base of the bowsprit. If I remember correctly (which is highly questionable), Revell represents each pair of hearts and lanyards as a single plastic part, which the instructions tell you to tie to the appropriate eyebolt. (You might want to consider replacing that part with genuine, individual hearts, from the aftermarket. If you do that, you'll probably also want to replace the molded plastic deadeyes and lanyards. I frankly don't recommend that approach unless you have some practice with such things, and/or want to spend a great deal of time modifying all sorts of rigging fittings in the kit. The molded hearts and lanyards don't actually look too bad.)
Seizing a line to one of those heart-and-lanyard assemblies isn't really difficult. Start by passing the stay around one of the hearts. (The real stay is made of wire, and it's thinner than one might expect: 5" in circumference. That works out to 0.016" in diameter on 1/96 scale.) Use some sort of gadget - an alligator clip, or maybe a small wood clothespin - to hold the bitter end against the standing part. (Those are fancy terms for the long part and the short part.) Take a generous length of the finest black thread you've got, and take a turn of it around the standing (long) part. Arrange the loop in such a way that the long part of the fine thread passes below the bitter end (i.e., closer to the heart). Then take the fine thread a dozen or so times around both parts of the stay. Finish up by taking one last turn around the standing part alone, and pass the bitter end of the fine thread under the previous turn. Pull the fine thread tight and, using a pair of tweezers, shove the turns together as firmly as you can. If you handled the first and last turns right, they'll be locked into place and the seizing won't tend to come loose.
Mr. Campbell's drawing shows three seizings on each part of the lower end of the forestay - about a foot apart. The first one may take you ten minutes. I'll bet the third one takes you about three minutes. Human fingers get trained surprisingly fast.
Now tie the other heart in that pair to the eyebolt, and glue the eyebolt into its hole in one of the knightheads. (Let's say the starboard one.) While you're at it, tie the other heart assembly to its eyebolt and glue that eyebolt into the hole in the port knighthead. Let the glue dry. Really dry.
Next, take the long end of the stay and pass it up through the "lubber hole" in the starboard side of the foretop. (That should be clear from the kit's rigging instructions.) Pass it behind the lower masthead and down through the port lubber hole. Pass it around the upper heart in the pair you mounted to the port knightead. Use the aforementioned alligator clip or clothespin to hold the remaining length of the stay against the standing part - but try to arrange the setup in such a way that the clip doesn't make the stay droop. You want the stay as tight as you can get it. Now rig the three seizings to the port part of the stay, just like you did the starboard ones.
While you're doing that, keep a close eye on the tension of the stay. A tiny bit of slack won't hurt; you've got one more trick up your sleeve to take it out. But if the stay isn't pretty taut at this point you'll have big problems later.
WIth the ends of the stay seized to the hearts, you still need one more seizing: the port and starboard legs of the stay get seized to each other, a few feet below the foretop. (If you've done the job right, putting that seizing on will take out any slack in the stay.)
Now put a drop of white glue (Elmer's or similar) on each of the seizings and let the glue dry. Then snip off the ends of the seizings, and the ends of the stay itself. For a neat, authentic touch, brush a bit of white paint on each of the seizings on the ends of the stay. (That's an interesting decorative feature of the real ship.)
The mainstay is a little simpler. Like the forestay, it runs in two parts; the lower ends pass on either side of the foremast, and are set up to iron fittings in the deck, under the fiferails just ahead, and on either side, of the foremast. If I remember correctly (highly doubtful), Revell represents those fittings as simple eyebolts. The real things are a little more complicated, but not much. For most observers those eyebolts will be plenty good enough. The upper end of the mainstay is rigged to the maintop, just like the forestay is rigged to the foretop. The legs of the stay are seized (with three seizings each) directly to those fittings, and the port and starboard legs are seized together just below the maintop.
The mizzenstay runs in one part. The upper end has a big eyesplice worked into it, the eye passing around the mizzen lower masthead. On a model I'd suggest passing the upper end of the stay through the lubber holes of the mizzen top and around the back of the masthead, as you did with fore- and mainstays. The lower end of the mizzenstay is seized (with two seizings) to an eye in the aft side of the mainmast, a few feet above the deck. (If I remember right, Revell represents that fitting pretty accurately.
The modeler can spend just about as much time modifying and improving that kit as he/she wants. One improvement I recommend highly is to replace the plastic eyebolts and belaying pins with metal ones. It's pretty easy to make your own eyebolts out of brass or copper wire. The aftermarket companies offer turned brass belaying pins; they aren't cheap, but they'll save lots of headaches. (Styrene plastic is a wonderful, versatile material, but it just isn't appropriate for some fittings on a ship model. I suspect some - maybe a lot - of the belaying pins in your kit were broken before you opened the box - and more will get busted when you start belaying lines around them.)
Describing stuff like this verbally is notoriously hard; it looks a lot more complicated in written form than it really is. (I could have rigged several stays in the time it's taken me to type this post.) I'll finish up with a suggestion I've made many times in this forum: if you want a really good source of advice on modeling the Cutty Sark, get hold of a copy of Mr. Campbell's plans. They're available, for a surprisingly reasonable price, from the ship's gift shop: http://www.cuttysark.org.uk/index.cfm?fa=contentShop.productDetails&productId=14853&startrow=1&directoryId=345 . I ordered a set a couple of years ago; the warning about the 28-day delivery time notwithstanding, mine arrived in North Carolina in about a week and a half. They would have been worth a month's wait - and then some. They're absolutely fascinating documents, and contain almost everything a modeler could want to know about the ship. One of the biggest bargains in model building.
Hope all that helps a little. Good luck.