There are multiple ways to add a border to a quilt top.  But if you plan to have a straight border and not a wavy border, then you need to add the border the RIGHT WAY!  Please check out a few websites that can give you clarification on how to do this BEFORE you add your borders.  It makes all the difference in the world.

Here are a few websites you can check out for directions:

https://www.thesprucecrafts.com/measure-and-sew-borders-to-a-quilt-2821965

Adding borders correctly.