Oooo, this in one of my favorite classes to teach because I know some secret tricks! LOL! Even my beginning students get excellent results with this method. Let me see if I can summarize it for you.
1) Before even trying to attach the borders, I fold the pressed quilt top NEATLY in quarters, raw seams inside, checking that intended center spots are really in the center. Then I line up the folded seams to both the top/bottom and then to the sides. This gives me a quick check that my edges are (within reason) roughly the same size as the center seams. I used to do this with the quilt unfolded and they were just too big. The weight of the big tops alone would skew the fabric too much to accurately check without a table to lay the monster flat! The results I get with the 1/4 fold are just as accurate. If the centers and edges are more than about 1/2" different net, I go back and look at the piecing to see if something is off somewhere. I do NOT proceed with the following steps if I am not happy with the overall squareness of the top itself!
2) To attach the borders, I fold the quilt in 1/2 only, raw seams inside again, and lay it across a flat surface so I can get to the center area. Lets start with the sides, so fold the top to the bottom. This exposes 1/2 of the length of the sides. I fold each border strip in 1/2 also, wrong side OUT and gently finger press to mark the center. (Note: Do each border strip separately.) I lay the folded in half border on top and down the center of the folded in half quilt top, carefully matching the center folds and gently smoothing the border out to the edge of the quilt. You should now have the folded smooth straight border, wrong side out, laying on top of and down the center of half of the folded quilt top. (Tough to describe! Hope I was clear enough.)
3) With the folded border on top of the folded quilt, turn back the ends of the border where they meet the raw edge of the quilt top. Again, gently finger press to mark these ends of the border. Do NOT trim the excess at this point. Fold each border edge back separately, not as a nested group. You have now marked the ends of your border.
4) Now take your border, with the folded 1/2 and folded back edges, and bring each edge fold up to the center fold and gently press these 1/4 and
3/4 marks. Do each side independently again, not nested or wrapped around each other! Your unattached border should now be shaped like a "W" or "M", not a "U". You should now have a total of five folds in the border, marking both ends, and the 1/4, 1/2 and 3/4 marks, measured exactly to of the center of your quilt top! Set your two border pieces aside.
5) Do the same folding techiniques on your quilt top, starting right side out and bringing the edges to the center. Don't forget to do the "M" or "W" folding, each side back to the center. Your quilt top will have three folds at the 1/4, 1/2 and 3/4 marks, and your raw edges are your edges! By folding the quilt top RST and the border WST, your folds will automatically nestle when you go to line them up for sewing.
6) To actually attach the border, place the quilt top and each border, RST, line up the folds and pin as necessary. Ease in any minor size difference between each set of 1/4 marks. There should not be many. You will find that by leaving the excess fabric on the ends, it gives you a nice straight fold to line up the edge of your quilt top to, which gives a more square result at the corners as well. Sew with your usual 1/4" seam allowance. Make sure to press your side borders as needed after attaching and then you can remove any excess on the edges using a nice square up ruler.
7) To repeat for the top and bottom borders, repeat from #2 above, but fold the sides together, including the newly attached side borders.
Where is the tape measuring you ask? Ha, silly boy! I don't need no stinking measurements! LOL, Sorry, a little punchy here. Personally, I have found that actually taking ruler measurements, writing them down, trying to divide them, etc., was just a bunch of wasted time I didn't need to spend. Does anyone really care if your top measures 37 and 3/16? Basically, if your quilt top edges are within reason to the size of your centers, both horizontally and vertically, and you have marked the individual 1/4, 1/2 and
3/4 locations on all of the sides to be attached, all will be fine. Your borders will lie flat and your top will be square.
HOWEVER, you still need to follow the basic principles of matching your center dimensions and evenly distributing any excesses. The above method covers those critical points. Some of the worst problems I have seen with borders is when they are not probably measured or marked. Even when you think that the edges are the same length as each other, if they don't match the center, (within reason) the borders will be wavy; if one quarter or, worse one half of the border fabric shifts while being sewn, moving more fabric to one end or the other, your borders will be wavy. Once you try the method I have outlined above, I think you'll be flying through those borders.
Good luck Lorraine in La Center