DIY Elastic Repair and Adding Sleeves - mending and altering thrift store finds

I love how frothy this dress was when I found it at the thrift store, but the elastic was dead and I like having sleeves more than sleeveless dresses. Lucky for me the elastic was super easy to change and I had a sleeve pattern that I liked and knew might work.

Changing the sleeves is a little advanced! Tackle it if you feel confident OR are not going to be heartbroken if it doesn't work.