Unlike some other kinds of needlepoint, there is no complex counting of threads or laying out of squares. You simply draw the "picture" on the cloth (unbleached and uncolored linen, if you want to be authentic) with a pencil (charcoal stick for authentic) and then do the outlines with a dark thread (black silk "fine Florentine".) The cross stitches which are used for filling in the spaces are in a slightly lighter single color: brown, red (sometimes brownish-red imitating faded umber), green, sometimes yellow (only moderate to dark umber silk is authentic -- this is Umbria, after all, the place where this brown earth dye was first sampled by the ancient Romans). Most importantly, the picture elements are what is "voided" -- if you are making a bird or a flower, the parts of the linen that are not bird or flower are what are filled, and inside the bird or flower the raw linen shows through.
How the stitches are done is also very important. A "correct" piece will look finished on both sides, with no tags or loose ends. When doing the black running stitches, they should be as small as you can make them, and should be sewn with a round-point needle. For really authentic work the needle should never pierce the thread of the previous stitch but rather should always lie to the same side of the previous stitch -- you probably need a loupe to see it, but it matters a great deal to collectors. The dense cross stitching should always be worked in the same direction -- left-to-right or vice versa -- unless a ridged effect (not authentic) is desired.
There were three known stages of development of this handicraft. It existed, and, according to tradition, was developed by the "Poor Clare" nuns during or slightly after the lives of the towns two famous saints, Francis and Clare. This is known because an artistic rendering of a piece of the work appears in one of the frescoes in the St. Martin Chapel in the lower Basilica of St. Francis. Simone Martini is thought to have completed the frescoes by 1317. The style of stitchery remained popular for about 150 years and then virtually disappeared. Except as museum pieces, work from this period is simply not found. The second phase was a revival in the 18th and 19th centuries, this time definitely spearheaded by the Poor Clares, who were looking for a handicraft to provide themselves and poor townswomen with an income. Much of the work from this period is also in museums or in private collections. The third stage, another revival in popularity, is going on today, both in Assisi as a source of tourist income and with needlework enthusiasts worldwide who obviously have a lot of time on their hands.
If you plan to buy Assisi embroidery on a trip to Assisi, you should remember the following. First, much of the modern work is very pretty and readily available in shops in town. Decide whether you want "authentic" (essentially brown or reddish-brown with black outlining). Bring a magnifying glass to look at the stitching: even if you don't know what you are looking at, it might impress the seller. Ask about materials -- is it silk thread? Is it on a linen base? Don't expect to buy real museum quality old work. That's all pretty much in museums (including the museum in Assisi). Don't buy anything that is said to be from the early periods unless you really are an expert -- in which case you don't need my advice anyway. Bring money or credit cards.
More Internet links:
Punto Assisi: http://crossstitch.about.com/cs/styleassisi/, and http://www.angelfire.com/zine/kiarapanther/embroidery/assisi.html.
Dozens -- hundreds more Assisi embroidery pages: http://www.google.com/search?client=googlet&q=embroidery%20assisi.
Links to this and other embroidery
and stitchery styles: http://crossstitch.about.com/cs/stitcherystyles/.