Sometimes a dress is just on the edge lengthwise-long enough to be covered, but not quite long enough to be comfortable. So many dresses these days don't really come with hems, but even a small hem can be let down to lengthen a dress.
This is my daughter's dress. It is one of those "long enough" dresses, but she wishes it was just a little bit longer to make her feel more comfortable when sitting and bending.
It has a small hem. After measuring the hem on this dress, I determined I could lengthened it by 3/4 an inch. That doesn't seem like a lot, but even 1/2 an inch can make the difference between "just barely" and a comfortable length.
I used a package of hem tape, which you can buy anywhere sewing supplies are sold. It is a lightweight flat lace.
Carefully unpick the old hem and press open gently. When you iron something, use up and down motion, not forward and backward. That way you won't stretch out the item you are ironing.
You can see that I left the edge finish on the dress when I unpicked the hem. This keeps the fabric from unraveling, so leave it if possible.
Pin the lace to the right side of the bottom of the dress, matching the edge of the lace to the bottom seam of the edge finish. This means the lace and the dress overlap about 1/4 an inch.
Now use your machine to sew along the lace, taking care not to stretch the dress fabric.
Now fold the lace and about 1/8 inch of fabric over to the wrong side and pin. If you have a hemmer on your sewing machine, stitch in place. I hemmed this dress by hand. There are many methods to hand hem, but I learned to hem by taking a small stitch through the lace edge,
then a tiny stitch through the fabric about 1/2 an inch away from the last stitch.
This leaves a kind of zig zag hem that will stretch with the dress.
When you are done, iron using a pressing cloth. You can buy a cloth from your fabric store meant for pressing or use a clean, thin, 100% cotton cloth (dish towel) like I am. A pressing cloth allows you to press without worry about scorching your fabric and creates more steam for better results.
Wet the cloth and wring out well, then place on the hem on the wrong side of the dress. Press, remembering to use an up and down motion.
It took me maybe 30 minutes to lengthen the dress. It didn't change the look of the dress, but it will now be more comfortably modest for my daughter to wear.