Several other people have asked this same question before. However, here's my $0.02
Unless you have experience sewing ballgowns and evening gowns, this will be difficult. There are some elements and techniques you may find complicated. This is not to deter you from doing it, just a statement of fact. Handling that many yards of fabric gets unwieldy. (Ever tried sewing 8 yards of fabric at once? XD My friend just did it for a set of curtains and was bitching and complaining the entire time)
For patterns, you should look at main commercial pattern brands before you ask here. Unlike other costumes, a ballgown pattern may be found in commercial catalogs. If you don't know, the brands are Simplicity, McCall's, Burda, and Butterick. Simplicity and Burda share the same parent company, as do McCall's and Butterick.