The first quilt I ever bordered, I did the thing that seems obviously correct: I laid the border strip along the edge of the quilt, cut it to match, and sewed it on. Both sides. The top rippled like a flag before it ever met the batting, and no amount of pressing improved it, because the problem wasn't in the pressing — it was sewn in. Wavy borders are one of the most common finishing problems in quilting, and the cause is almost always the same: the border was cut to the length of the edge, and the edge was lying.
Why the edge lies to you
By the time a quilt top is fully pieced, its outer edges have been handled more than any other part of it. Every seam that ends at the edge has been pressed there, every row has been dragged across a table by it, and the outermost inch of fabric has stretched — a little or a lot, but it has stretched, because there's nothing beyond it holding it stable the way the interior of the top holds itself. Measure the edge and you're measuring the quilt plus that stretch. Cut a border to the stretched measurement and sew it on, and you've made the stretch permanent: the border is now genuinely longer than the quilt it's attached to, and the only place for the extra length to go is up and out of the plane of the quilt. That's the ripple. It won't quilt out, and it gets worse at the next border if there is one, because each new round is being measured against an edge the previous round already distorted. The fix costs nothing but a tape measure and a different habit.
Measure through the middle, in three places
The interior of the quilt hasn't stretched the way the edges have, which is why the honest length of the quilt lives through its middle. To cut side borders, measure the quilt's length vertically through the center, then again roughly a quarter of the way in from each side — three measurements down the body of the top, none of them along an edge. They'll usually differ by a little; average them, and cut both side borders to that averaged number. Both borders, the same number — that's what pulls the quilt square as they go on, rather than preserving whatever asymmetry the piecing left behind. If one of the three measurements is wildly unlike the other two, don't average it in blindly; re-measure first, because tape sag and fabric shift produce fake readings more often than quilts produce genuine three-inch bulges.
Cut on the lengthwise grain when you can
Fabric doesn't stretch equally in every direction. The lengthwise grain — parallel to the selvage — has almost no give, while the crosswise grain stretches noticeably, and anything cut at an angle to both stretches most of all. A border cut on the lengthwise grain is as stable as that fabric can be, which is exactly what you want in the piece whose whole job is holding the quilt's edge to a fixed length. The tradeoff is yardage: lengthwise borders for a large quilt need a continuous length of fabric as long as the quilt itself, where crosswise strips can be pieced from far less. When yardage forces crosswise strips, they work fine — join them with diagonal seams and handle them gently once cut, because a stretched border strip has the same problem as a stretched quilt edge. What I'd avoid is mixing: two lengthwise side borders and two pieced crosswise ones behave differently under the needle, so if you mix, expect the crosswise pair to need slightly more care easing on.
Pin the ends, then the center, then ease
A border cut to the averaged middle measurement will usually be slightly shorter than the edge you're sewing it to — that's not an error, that's the method working. The border is the correct length; the edge has extra. Your job at the machine is to distribute that extra evenly instead of letting it pile up at one end. Pin the border to the quilt at both ends first, right sides together, matching the border's ends exactly to the quilt's corners. Then find the center of the border and the center of the quilt edge — fold each in half and finger-press a crease if you don't trust your eye — and pin the centers together. Now halve each remaining section the same way, pinning at the quarter points, and keep subdividing until the fullness between pins is small enough to manage. Between every pair of pins there'll be a whisper of extra quilt edge; sew the seam with that fullness distributed flat, and the finished border pulls the edge back to the quilt's true length.
Then do it all again for the top and bottom
Once both side borders are on and pressed, the top and bottom borders get the same treatment, with one detail that's easy to miss: measure the quilt's width through the middle in three places including the side borders you just added, since those are part of the quilt's width now. Average, cut both strips to that number, pin ends-center-quarters, ease, sew. Sides first, then top and bottom, is the usual order simply because it means only two of the four strips need to span the full final dimension — but whichever pair goes on second must be measured after the first pair is attached, not from the pattern's stated dimensions.
When the two sides genuinely disagree
Sometimes the three-measurement check tells you something you didn't want to know: the quilt's left half and right half really are different lengths — not tape sag, not a misread, but a genuine half inch or more of disagreement, usually from seam allowance drift accumulated across one side's blocks. For a small disagreement, up to around a half inch, the averaged border handles it: cut both borders to the average and the longer side simply gets eased a little harder, with its extra fullness spread across the full seam where it disappears into the quilting. Beyond that, easing alone starts to show — a border can absorb only so much before the edge looks gathered. First check whether the difference is concentrated somewhere specific: one block that finished long, one seam sewn scant. Unpicking a single seam and restitching it a needle's width deeper, in the middle of the offending row where it won't be noticed, can remove most of the discrepancy at the source. If it's genuinely spread everywhere, deepen two or three interior seams by a hair each rather than one seam by a lot, then re-measure and border to the new average. What I wouldn't do is cut each border to match its own side — that locks the skew in permanently and hands the problem to the binding, which has even less capacity to hide it.