Hi,
A few tips I learned from a couple of girls who sew custom drapes and slipcovers. Always use straight of grain/ the selvage length for borders even if you have to piece it. Measure across the middle and use that length even if it is too long or to short. Which ever side is fuller, border or top put on the bottom when sewing and let the feed dogs ease it in for you. It is kind of like setting in a sleeve if you have ever done that. The feed dogs will ease in an amazing amount of fabric.
Good luck. We all have made tops that ripple. I know measuring is a pain but it is easier than ripping and tearing our hair out.