oh my...I hear ya..it can be intimidating on such a large quilt...BUT...make you a practice sandwich...pillow size (16") and just baste it all on...paying close attention to how you lay those corners....
if it works you have a visual reference..if it is still ugly, pull the threads and go again til it does work....you CAN do this....corners are my down fall...I have finally learned to cut some of the batting out on the edges before trying to miter my corners!