IMHO, the best source on mid-19th century sewing is Elizabeth Stewart Clark's
Dressmaker's Guide. It's available through her Sewing Academy website. It includes recommendations for patterns, how to design your own, and how to alter for variations in the basic pattern, such as different bodice, neckline and sleeve styles. I agree with
@NurseErin that Past Patterns is one of the best. Past Patterns instructions are often a sewing manual in themselves. I have been told, however, that some of the most recent Past Patterns issues may have problems with fitting because of using computer formulas to mark the sizes. Just because a woman has a larger bust or waist doesn't mean she has a larger neck, for example. It's really important to cut out the pattern in cheap fabric (inexpensive muslin, usually), baste it together, fit it to your own body (with help), and trace the pattern with revisions for your own shape on separate paper (leaving the original pattern intact) before cutting the fashion fabric.
Also, unless you are working with post-Civil War fashions, you don't need a pattern for the skirt, only the bodice. The skirts for 1840-1865 were made from straight panels. Just calculate what is needed, measure, and rip! My own recommendation is to calculate at least three times and measure at least twice, since you can rip only once!
The
Dressmaker's Guide also has instructions for sewing all the necessary underpinnings: chemises, drawers, corset, petticoats, and, for 1856 on, hoops. Patterns aren't really needed for these, although for a corsest you may have better luck with authentic corset patterns (Laughing Moon is often recommended for their two corset patterns) than with the methods that Liz Clark uses. (I found that the duct-tape pattern method she desribes works better in theory than in practice, at least for me.)
The problem with the modern pattern company versions of Civil War era dresses are that they do not use 19th century fitting and sewing techniques, so the resulting garments often don't look authentic. A number are designed as stage costumes rather than garments to be worn in the field. A few, however, were designed with the help of true designers of CW reproductions, such as Kay Gnagy.
It's also important to familiarize yourself with the fabrics and prints of the appropriate era. Unless you're really lucky, you won't find those fabrics in a local chain store. Many of us mail-order our fabrics, usually ordering a sample to check out before purchasing the total 7 1/2 yards needed for a dress. There are a number of good sources on 19th century fabrics, from the most recent--and extremely expensive--
Wearable Prints 1760-1860 by Susan Greene, to my favorite, Eileen Trestain's
Dating Fabrics. I was fortunate to attend a half-day workshop session with Eileen two years ago in which we examined her extensive quilt collection.
You may find workshops being held in your area. Watch for those and be sure to attend! In my own area we also have two different custom seamstresses of 19th century garments who hold classes.
There's nothing wrong with using your sewing machine for garments from 1855 on, when machines sold on the installment plan became quite common. However, remember that the machines of that era did only straight stitch. You do need to master hand-sewn buttonholes. I'm still working on this, but already find that even my rather uneven products are so much more durable than machine-sewn buttonholes. You can avoid the buttonhole issue by using hooks and eyes, and using buttons just as decorations. With this method you can put the hook-and-eye closures closer together at points of stress, while spacing the buttons evenly. This was a common 19th century technique.