Honestly? On a wing and a prayer. I originally began by using the graph paper years ago. But, now I have gotten pretty good at knowing what it will look like when completed. I do audition fabrics as I go with some of them. Because something doesn't quite set right with me. But, I like that creative way of making them. It frees me to Do My own thing.
Many of the bought patterns I use for quilts are very complicated and take days to complete. So when I get the chance, I play like this or make a purse. I have be creative once in a while or I get to feeling like the humdrums have set in and I am a slave to the piecing and quilting. Know what I mean?