A friend of mine needed sleeves added to a dress she was wearing to wedding. This was the right color, just no sleeves. I hunted all the local fabric resources and found some fabric that was the same color at JoAnn Fabrics. It was knit, so I was happy because that's what I work with mostly right now with my nursing shawl business.
First we determined how far down from the top of the armhole we needed to make the sleeve.Then I measured the total circumference/length of the armhole. I always like to add extra length to allow for 'just in case'. I made a rectangle with those two measurements plus seam allowances. Then I put the dress on a dress form and pinned the 'sleeve' into the dress. I had her try it on and then sewed it on top of an existing stitch line. It was pretty simple.
Then for the center, I added a square of fabric sewn on one side and a snap on the other (so she could pull it on unsnapped.)
This is the mother of the bride's dress which also needed sleeves.
I added pleats to the sleeves to echo the design of the skirt.
This is the rectangle sleeve with pleats sewn after being pinned. I trimmed off excess with pinking shears.
The sleeve pinned into the dress. Notice the simple pleat.