Right from the beginning, I worked on graph paper to design and I tend to think that my Welsh ancestors would've used it too. So, to get started, I ask myself a couple questions. How big will it be, how many borders and so on. When I work on graph paper, I can figure out options for enlarging it. For example, if the corner blocks are 4x4 squares on the graph paper, I can figure that one square will equal 1" and then figure the square to be 4"square and so on. However, the best way to get started is to figure the overall outside dimensions and work towards the inside.
This picture shows a page in my sketchbook with the basic design for the quilt I'm working on at this time. This give me an outside dimension, the width of the outside borders, an additional set of borders because the quilt is a rectangle and the circle in the center. I will often try out different patterns in different spots. Sometimes I use what I sketch in but often times, the design takes on an organic nature and a life of it's own but the design on paper is still a good way to get started, even if you don't stick with it. You can see where I've written notes around the outside which will help me remember ideas. I've also included the double lines (lower left corner of sketch) to tell me which side of the dividing lines I plan to put the lines which are a key feature of Welsh quilts. The double lines define the borders, ect. Resist the temptation to leave them out, they define the design and are worth the extra marking and quilting. The double lines are also helpful in defining any pattern that you want to stand out a bit such as a leaf or heart. Patterns not double lined tend to blend together... more on this later.
So, go make sketches on some graph paper, choose your fabric and get ready to start marking your top.