You can find patterns for Cathedral Windows all over the internet, simply by tapping "cathedral windows" into your search engine! I suggest you read quite a few, and then save and/or print out the one you like the best. All will work, and some are simpler than others. Some are 100% hand-stitching, and others are a mix of machine stitching and hand-stitching. I find it handy to make stacks of the basic "before you add the windows" bits by machine at home, and then cut the "windows", and put them into plastic zipper-style bags to finish later. I take them with me when I travel and am in the mood for some hand-stitching. Enjoy!
PS. Before you invest in lots of background fabric, I suggest you make 9 of the background blocks, assemble, add the windows, and see whether you enjoy working on this pattern! You will have a sample, have experience in the work, and know whether you want to make something large, all before you spend much money! My sample piece is under a cut glass pitcher full of silk flowers in one of the guest rooms.