When I started working with Welsh quilt patterns, it took a while for me to relax and just draw spirals without trying to make them all perfectly round or matched to each other. In the early 90's, quilting stencils were still in wide use here in the States and American quilting was overly dependent on them. I had used the stencils but didn't like them because of the limitations of the stencils and this was a big part of my attraction to Welsh patterns... they seemed so free.
Over the years, I've generally marked patterns two ways. The first was to make my own custom-fit stencil and the second was to just free-hand mark patterns. In my early years, I mostly used the stencils, partly because it speeded up the marking but still looked like a hand drawn pattern. When I was teaching workshops, it was much easier for students to adapt to stencils because they also had stencils in their quiltmaking background.
The basic steps for making a stencil was to draw the design onto template plastic with a black Sharpie pen and then cut along the lines to leave a gap big enough for the fabric marker to get in and mark the pattern. I used narrow strips of masking tape (on front and back) to bridge the gaps and stiffen the stencil. Sometimes, I did a whole pattern but often I marked the whole pattern but only cut and taped half of it. It isn't the way the original Welsh quilt patterns were marked but it works.
My favorite way to mark patterns is by hand. I do the dividing of the main parts of the design with rulers but I've come to really enjoy the hand marked patterns which are more consistent with the old quilts.
In the picture below, part of the patterns were marked with help from stencils and some free-hand. The spirals were marked free-hand with a little help with the spacing.
The picture below shows how a circle can be divided up for spirals or any other pattern. I took a sheet of printer paper and cut a basic curve for a quarter circle and then folded the circle into 3 sections from the corner out (the folds barely show in the picture). The folds are nipped at the ends to help with seeing them and the remaining square corner of the paper is positioned into the center of the circle and the circle border can be divided up for the spirals.
Now the curved border has been marked using the paper shown above and a ruler was used to finish the marking the spaces.
This picture shows how the spiral was marked with soap but the quilting doesn't have to exactly on the marked line.
Another tip for spirals is to know which way you prefer to hand quilt spirals because some quilters have a definite preference. You can try marking samples of spirals going both directions and see if one direction is better than the other. If you like clockwise spirals better than counter-clockwise (like the ones above) then it'll save you time to mark them for easier quilting, The best way is to be able to quilt in any direction with different fingers.
May you have many happy hours quilting spirals.