I'm not best placed to give any advice about how to add borders, I'm afraid your friend wouldn't approve of my, grab the fabric and go for it method, but I would say to you that I think your mentor is perhaps now hindering rather than helping. If you've made the tops then you're ready to add the borders. So what if you make some errors in the process, its only a big deal if you allow it to be one. You're in charge, not your mentor and tbh unless you bite the bullet and get on with it, you'll be stuck thinking you can't do something when my guess is you can do it perfectly well. Measure twice, cut once, is a good place to start. Watch the videos, and you will fine. And out of sheer damned cussedness on my part I'd make those borders with mitred corners
