Tie the Kevlar around the motor tube, and then pass it through a hole in the top centering ring or through a small notch cut into the edge of the ring. Make it just long enough to reach the top of the tube with none extending outside of the tube. Then tie your shock cord to it. Kevlar is very abrasive and has absolutely no stretch, so if any extends beyond the top of the tube, it can really mess up the edge when the recovery system deploys. In order to keep glue off of it when you go to glue the motor mount in, take your Kevlar/shock cord combination and pass it down through the top of motor tube and out the aft end before you insert the mount into the rocket. After you have bonded in the motor mount, you can push the cord back out the front of the motor tube and up out the top of the airframe.
The idea with using a Kevlar "leash" for your shock cord is that you can anchor the cord deep down in the tube, below the parachute. Because Kevlar won't burn, you can use it for the first section of your shock cord system - the part that is always in the airframe - and anchor it down near the top of the motor or around the motor mount itself without worrying that the ejection charge will burn through it. The traditional paper mount requires anchoring the shock cord up near the top of the tube so that it is as far away from the motor's ejection charge as possible. But with that arrangement, the parachute and most of the shock cord is below the anchoring point prior to deployment, and it all has to get past the anchor on its way out of the tube at deployment time. This can sometimes cause a bit of a traffic jam, resulting on occasion with an incomplete deployment or even a failed deployment. By anchoring the shock cord low, the anchoring point and most of the cord is below the recovery device prior to deployment, and there is nothing above the recovery device to get in the way at deployment time (except the nose cone). The recovery system can simply extend out of the rocket at deployment in a straight line, without having to make any U-turns.
High power rocket fliers usually don't have the same issues with where to anchor the cord, because most of their designs allow for much more customization of the location and method of deployment of the recovery system. But Kevlar (usually in a tubular weave) is still a popular choice with them more for its strength than for its resistance to high heat, although that is usually an important consideration as well.
Kevlar line comes in a variety of forms and is usually identified by its test strength or breaking strength. Some amazingly strong cord can be found in Kevlar line that is quite small in diameter. Common forms of the cord are Kevlar thread (very fine strands twisted tightly together, just like sewing thread), Kevlar string (similar to thread, but larger in diameter), Kevlar twine (two or more "strings" of material - each composed of tightly twisted individual strands - that are then twisted together along with a bonding agent), braided Kevlar cord (fine strings of Kevlar are incorporated into a complex braid, producing a very strong cord) and tubular Kevlar (the strands are braided together in an open tube format, which allows for larger diameters of cord with nearly the same strength as solid braided cord but with less weight, and which may also give the cord a slight amount of stretch).
Kevlar cord is hard to cut; you will need a strong and sharp pair of scissors or shears or even wire cutters to cut it. You will almost never find it at your local hardware store or home improvement center, or even at many hobby shops. Fortunately, it is readily available online.
MarkII