Many quilts you see at shows now use faux trapunto which means using two batts. I have experimented with this, but just don't like how stiff it makes the quilt feel. I think it is worth the effort to do machine trapunto.
The technique involves soluble thread and is described by Hari Walner in her book Trapunto by Machine which was published in 1996. She was very clever to figure it out!
I start by loading the trapunto batt on my longarm and pinning the top to it. The white thread is the soluble thread - I plan to use several colors of thread when I quilt and the white thread will disappear when the quilt is washed.
I made several stencils to fit the blocks and used them to mark with white chalk as I advanced the quilt.
I'm using a wool batt for the trapunto and it has the advantage of being slightly transluscent so I can see the blades of the scissors through the batt. It helps me avoid burying the tip of the scissors in the top and accidentally cutting it.
Once I had sewn around all the areas I wanted to have pouffy, I took it off the machine and started to trim around the trapuntoed motifs. This is time consuming and SCARY because it is so easy to snip the top. (Lynn if you are reading this - I"M BEING VERY CAREFUL) I have to use sharp little embroidery scissors to make it possible to get in close.
Here is the top after four hours of snipping (this will be inside the quilt, it is the back of the quilt top):
After nine hours:
After thirteen hours:
Another two or three hours and I will be done. Then on to the next step!