The Secrets of Writing with Quills
First, I must state that much of my knowledge of quills and writing comes from two individuals, Jim Downey and Jim Daniel. the rest I have picked up with hard experience writing with these things, which I can do passably but not nearly as well as the aforementioned gentlemen.
It seems obvious, but the better the quality of quill, the better you can write with it. The point must be hard - quills were hardened by plunging them in hot sand after cutting them. Quills don't last forever. In the Civil War, 12 quills were issued every quarter as part of the normal stationery issue. So, simple math shows us that a quill was expected to last about a week in normal administrative use.
Another hint is that quills were cut in two distinct ways, and one style is much easier for a beginner to use than another. Some quills were cut to a point and then split, so that there is a channel running down to the point. This is the method that most of us are familiar with. This is also the most difficult one for beginners to write with, as too much pressure on the point when writing widens the split and causes all of the ink to descend on the first letter. The other method of cutting a quill is scoring the point area with many fine cuts all leading down to the point. This method does not split the point in two, and allows slightly more pressure when writing without dumping the ink onto the paper. I suggest that those of you who want to begin writing with quills start with the scored ones.
The Writing Pressure
LIGHT, LIGHT, LIGHT!!! As Jim Downey told me, hold the quill as if writing and make small circles with it on the back of your other hand (Try this without ink. :) ) When you can make slight indentations on your hand without leaving a slight scratch mark, you are using the proper pressure to write. The pressure most of us use to write with today is way, way, too hard to use with a quill.
Use a good calligraphy ink. DO NOT USE INDIA INK WHEN USING A QUILL! India ink has too strong an adhesive quality, and will not flow freely from a quill. When using powdered ink (I shamelessly recommend my own, mixed especially for quills.), remember that it will not be a true black, but more of a dark gray. Actually, I recommend that you do not use powdered ink when starting out. Go to an art store and ask for calligraphy ink.
Many of us want to use laid paper when we have gone to the trouble of writing with a quill. The laid paper commercially available today (generally called either laid paper or resume paper) is textured on one side only. My advice to you new quill users is to turn the paper over and use the flat side when writing. It is a difficult thing to balance the light touch needed and the ridges in the laid paper. Experience will allow you to turn the paper over and use the other, textured, side.
The Writing Style
They didn't write in the 18th century like we do today. (Oh, and by the way, not everyone had great handwriting I can tell you.) They wrote differently, using a roundhand style which generally is called copperplate today. The reason that they wrote in this manner is that a quill pen and free-flowing ink are very hostile to abrupt changes in the direction of the pen. If you think of it as painting a fine line on the paper rather than writing, you will get the idea.
There is no substitute for practice. When you starting writing in school, it was tough at first, your hand got tired, and the letters never seemed to look right. After a great deal of practice, you got better. Writing is a skill. Any skill improves with practice. Good luck.