Serena Dyer has just written a wonderful article showing you step-by-step photographic instructions making an 1812 muslin gown based on an example held at the Museum of London.
This article is on that wonderful, resource-full website, Your Wardrobe Unlockd. Yes, you do need to be a member to read the articles, but I believe it is worth it. I am constantly visiting the site and learning more all the time about historical costuming. And if you want to take your costuming a little further, there is a sister site, Foundations Revealed, where you can learn all about making corsets and other undergarments.
...added from comments below....Wardrobe Unlocked and Foundations Revealed are offering subscription deals- buy one month- get two free. Seems like enough incentive to give it a try!