I centered my stove pipe in the roof ring with tie wire. There's no way that pipe will ever shift.
As for the China cap, I got a ~3'x4' sheet of flashing at Home Depot for $5. It's in the aisle with the vent pipe. *Note: vent pipe 'IS NOT' stove pipe!* Mark a 5" d circle in the center with a Sharpie. Cut a slit to the center, and cut out the circle. This is a real trip with straight cut snips, lemme tell ya.
Overlap the edges until you form a cone. Eye ball the cone shape while eyeballing the roof pitch. Temporarily tape both edge seams. Cut a circle around the base of the China cap, about 10" bigger than the yurts smoke hole diameter. Keep the 5" hole in the middle when you lay out the circle. This is also a trip. Laying the cap on a flat surface helps mark where to cut. It's all by eye. Remember to start big cause it'll shrink in dia. as you refine the circle.
Set a ladder under the smoke hole and shove that sucker up through the roof ring and center it over the hole. You have to bend it to get it through the hole. Go out and eyeball the angle. Adjust as necessary. Make witness marks when finished.
To secure the conic shape I used 8-32 x 1/2" round head nuts and bolts along the two overlapped seams. I used four bolts on each seam. Do NOT put tinners metallic tape on the seams. The adhesive will stink up the yurt when it gets hot. Guess how I learned that one. lol
You want the cap to slide over the stove pipe so remark the -now ovalized- 5" hole some 1" larger in dia. than you double wall stove pipe size. Try the pipe for fit. You want the hole bigger, and round. Have more fun with those straight snips.
To finish the outside edge of the cap, snip tabs all around the outside edge. Make your cuts about 1" apart. Bend the tabs UP, towards the sky, not down. Pound them flat. China cap is now basically done.
I tie wired my cap to the stove pipe guy wires. Drill a pair of 1/8th holes about an inch apart in the cap. Two pair 180 degrees opposite eachother, with the stove pipe hole in the center. Loop tie wire up through a hole and back down through an adjacent hole. Secure in place to the pipe tie wires of whatever. It won't flap up and down in winter wind.
In the summer the tie wire is off. I jamb it up on an angle to the stove pipe with a stick so the breeze flows out. If it gets windy and rainy I bop it with the stick, and let it flop down flat. Cheap. Ugly. Safe. Works great.
Have fun.