I've been sewing since I was four years old. I learned first by my aunt. She just gave me some fabric, thread etc, a pattern and showed me how to work her machine and then said go at it. Will I looked at the pattern and realized it was a lot larger than I was. But Auntie, it is too large for me. So fix it, she says and runs out back to do her thing in the garden. Will you know I figured it out and made a garment that fit me. I don't really recommend that way of learning. It can get really frustrating for people and they give up all together. But, it tells you that you can adjust ready made patterns to fit your needs. You can adjust them to be modest.
Every weekend I go to Jo-Ann's and look at the pattern books. I've noticed that they are have some more modest patterns than they have had for a long time. They are having a lot of long skirts available now. And making a skirt longer is easy. I recommend getting pattern tracing material as it makes lengthening a skirt so easy. I actually copy my whole patterns over onto pattern tracing material. I get my tracing material at http://www.fabric.com They have the cheapest that I have been able to find and they have free shipping for 35 dollars. In fact, fabric that I can not get from my friend at http://www.gracefulthreads.com I get from fabric.com. But one of the new fashions that I like is the flared skirt. Also, they are having reproductions of older patterns like the 50's. Not all 50's patterns are modest but some are. Also, if they are too low one can use an under tee shirt or make a modesty panel to cover the area. Sleeves like hemlines are usually easy to extend. Just measure your arm from top of arm to where you want it to be in length and don't for get to add for the hem a couple of inches. Also realize that if the sleeves get narrow at the end you will have to fold the tracing material up and cut it out so that when it is folded up it get wider to conform with the sleeve. Sleeveless dresses can be easily turned into jumpers by putting a blouse underneath. They still have plenty of modest blouses to choose from. Just make sure to measure yourself to make sure you get enough ease in the garment. You want enough ease or more fabric above your measurements to make it not be clingy. I personally wouldn't use strapless dresses as jumpers because they normally have a bra top and that to me is too much bringing attention to the bust area.