When sewing border pieces on, are you measuring and pinning? I’ve had 2 newbie quilters who just sewed the borders on w/o measuring or pinning and the result was wavy borders.

Wavy edges are mostly caused by one of two things: 1) bias edges and 2) excess fabric.

Let's tackle bias first, and please forgive me if this is stuff you already know. It's kind of hard to figure out exactly what the issue is with the scant info we have regarding your quilt (it would be helpful if you could post pictures). Fabric can be very stretchy on the bias, which is across the diagonal. If you have lots of triangles in your quilt, blocks that are set diagonally, or setting triangles that are cut on the bias, your quilt may become stretched and wavy. Ways to prevent this are to starch fabrics before you cut them, cut setting triangles so the bias is not on the outer edge of the quilt, and handle anything with bias carefully. (Setting triangles are the triangles that are on the outer edge of a quilt with blocks that are set on-point.)

The other issue may be that of excess fabric. If your quilt has borders on it, you have to be careful when sewing the borders on. If they're too long, or you or your machine is pulling the fabric too hard or unequally as you sew, you'll get wavy borders.

If you post a pic, we might be able to help diagnose where the issue is and come up with solutions.

Instructions for how to post a picture, I stole this :D from fellow member OurWorkbench (Janey & John):

We like pictures. In order to post pictures on Quilting Board, you will probably need to reduce or compress the picture to a file size that is smaller than 2MB. How to post images can be found at https://www.quiltingboard.com/attach...020-01-17-.pdf
Don't forget to scroll over to the far right to find and click on the "Upload" button.

I have found that only the big red "Reply" button or "Quote" work to give me the paper clip icon to work for adding pictures.
https://www.quiltingboard.com/attach...p-location.jpg

Some additional info regarding reducing images at post #4 and images for the scroll and upload button can be found at #5 of quilt block ID?

Another way to post pictures can be found at 2021/2022, 2.5” Birthday Swap
And how to use an ipad to post pictures can be found at Uploading photos

Or simply - Make sure pictures are smaller than 2MB
Click on red "Reply" button
Click on the paper clip icon
Click browse - choose picture/s. (only 5 allowed per post)
Once they show up as being loaded, Scroll over to the right side and click on "Upload"
After the pictures show up as being attached, you can close that window and go back to the screen to finish posting your post.

if you are finding your inidividual units are wavy in your pieced blocks, it is probably due to
1. fabric is cut on the bias
2. poor ironing techniques

Your thread tension may be off.
by that I mean top and bobbin tensions mismatched.

Glue roller
 

[QUOTE=Onebyone;8567953]I make sure the quilt sandwich is basted with no give at all. I use Elmers school glue. My quilt lays flat as paper. When the glue dries it does shrink and pull the fabric. I simply iron it and the glue will not reshrink. This glue roller bottle makes glue basting fast and easy. Thanks to BonnieJP for posting about this wonderful way to apply the glue.
https://www.woodcraft.com/product_im...g?c=1515740567
Where does one find such a roller to apply glue to a quilt? I've heard others say school glue is the way to baste quilts.

I prewash all my fabrics because all the chemicals that are in them give me problems. As a result, my fabrics are floppy. I spray starch them so they are very stiff - that way even if I am cutting on the bias there is no movement when I piece and very little when I quilt. Since I mostly use an accuquilt die cut machine, the stiff also means nothing moves when I cut and if the grain is a bit off it doesn't matter.