My first advice is use enough thread! A Cluny holds it shape when it can't do anything else. The thread that is woven should be fairly packed in. In an exercise to see how much of the weaver thread is used to weave a Cluny, I got about a foot of thread woven in a Cluny on average. The larger the thread, the less you use, generally, but not by much, maybe an inch or so(30 cm).
The frame that you make the Cluny on, whether in your hand or on a tool, should always be under tension. The tension to increase or decrease the shaping is on the weaver. Don't collapse the frame in an attempt to change the shape of the Cluny. It doesn't work.
I count my rows as complete round trips from the start on the right side of the Cluny and including the return from the left side. On slippery threads I start the Cluny with an unflipped first half of a DS, and then start the weaving. Right to left is Under-Over-Under and Left to Right is Over-Under-Over.
My mental image, which I learned in bobbin lace although I learned tatted Clunies first, is to begin by making a triangle, starting at the tip of the triangle and widening as the weaving progresses. Make a straight square section about the length of the triangle and end with a second triangle that tapers to the end point of the Cluny.
<> vs. <>
Shaping the triangles obviously shapes the Cluny. Without the straight section, you get a diamond, which is a trick worth remembering, but for a leaf, there needs to be a break between the triangles. The straight section gives the illusion of a curve and produces plump leaves.
I count my round trip rows as I make the first triangle so that I can use that count for the straight section and the second triangle section. On average I use 4-6 round trips per section (that foot of thread has to go somewhere!).
Shaping the triangles is accomplished by increasing or decreasing both sides by the width of the thread. When I reach the left side, I adjust the thread so that the turn around the right side of the frame is right where I want it to be. When I reach the right side, I adjust the turn of the thread for the left side. Each time the weaver turns around the frame, the previous turn gets locked in because now there is nothing pulling on it.
It is okay to allow the weaving not to pack down as you make it. As long as the shaping is progressing to your satisfaction, packing it down will not change its shape. You may want to pack down the first triangle before starting the straight section to see if it is big enough.
The ending of the Cluny is a make or break operation. Finish weaving the ending triangle on the right side and then bring it up under the right side of the frame and stop.
Do not let there be ANY tension on the weaver. Lengthen it and place the shuttle in a safe place so nothing can pull on it. Do not release the tension on the frame until the weaver thread is safe.
Pack down the rows of weaving toward the beginning of the Cluny if necessary.
Release the threads of the frame.
For frames made in your hand or on a tool there is a loop above the end of the Cluny and a loop below the beginning of the Cluny.
n Loop above the end
^ Last triangle
 Square section
v First triangle
u Loop below the beginning
Firmly hold the woven Cluny between the fingers of your left hand. I call this the clench instead of the pinch because you DO NOT want the woven thread to move.
Gently pull the lower loop to see if you are reducing the upper loop. If not, pull the other side of the lower loop. Continue pulling on the lower loop until the upper loop is closed.
Now pull the ball thread at the top of the Cluny until the lower loop is closed. (Are you still clenched?? Good.) Release the Cluny and admire it.
Using the frame made on your hand or a tool, a Cluny is like a split ring that starts at point A and ends at point B. This, of course prevents hanging Clunies.
Ruth Perry's brilliant new Cluny method allows Hanging Clunies or changing colors, but cuts either the weaver thread or the frame (ball thread) depending on which end of the Cluny you want to continue from.
.B Cut frame thread continues with shuttle weaver thread
.O Loop to pull the cut end of the weaver thread to A
/ \ Tails of the frame made by the Ball thread
.O Loop to pull the cut end of the frame thread to B
.A Cut weaver thread continues with the frame thread
First the loop above the Cluny is closed by pulling on the right tail of the frame thread.
Then the cut thread is pulled up or down through the center of the Cluny as desired. The weaver is cut for a hanging Cluny and pulled down through the woven Cluny.