Linda, if necessary, you can use an electric knife to perform 'surgery' on the UY form. Many, many years ago, one of the ladies in a class I was in had to do a double mastectomy on hers, then reduce the size and glue them back on in a different position. Another had to slope the shoulders more.
In addition to a helper, I would advise a full length mirror. Also keep your digital camera handy for side by side shots for comparison. (We didn't have that tool 35 years ago!)
If you search PR, there are some excellent posts by Debbie Cook, made during and after she customized her form (not UY). She also put up photos on her website. In the Tips and Techniques, Kathryn has posted an excellent tutorial on customizing a dress form. Hrs was also not a UY. Nevertheless, both made a number of excellent points that apply to any dress form.
If you decide to use your own software sloper as the base for your cover, it must be a shoulder princess, with CF/CB seam/opening, and it needs to be mid-thigh length. Whether you make your own cover or use the one from UY, after the cover is pin fitted to you, so that it is as tight as a second skin, stitch it a few threads tighter on each of the lengthwise seams when you sew it up. There will be some slight stretching caused by the foam and any additional padding. Oh, and if you make your own, you'll need to add some darts from CF to apex unless you prefer the uni-boob look. heehee
A dry cleaner's bag is handy to use over any padding when you are putting on and taking off the cover during the process. After you've got the padding to the point that it's duplicated your figure and posture, you can use strips of that lightest weight fusible non-woven interfacing to hold the padding in place. Just lay the strips in place and press with your iron. Then use the dry cleaner's bag again (make sure it's completely open at top and botom), slip the cover on, zip it up most of the way, and then pull the dry cleaner's bag out the botom.
My mentor and teacher actually preferred that our forms be a tiny bit larger than each of us as she was aware that most beginners tend to fit too tightly on a form, neglecting the need for eae.
Almost forgot - if you do use an electric knife, do NOT ever use it for food! Best of luck with it, and do keep us advised of your progress, with pictures. We all love pictures
Julia in Houston