Don't fret too much about costume. We want it to look somewhat realistic and accurate to the time period, but we're not going to get too pedantic about it. Women, don't worry too much if you can't find that perfectly medieval gown. Chaps, don't worry - we're not going to be so cruel as to make you wear hose if you don't want to. You should carry a knife/dagger however. Women, if you're playing a widow or a wife you should be prepared to at least cover your hair with an opaque veil and if you're feeling really re-enactory then you could go for a wimple as well - though that's accurate to the period we're not going to make you. That would be as mean as making the men wear hose. Also try not to fret too much about footwear and don't, unless you're really into authenticity and accuracy, worry about getting your hair cut or extended. Sensibilities regarding the carrying of rosaries will be respected and such will not be enforced.

Here's a list of some sewing patterns which might be useful:
Butterick: B5656 (good mans pattern - look is authentic)
B3552 (a little too unshaped but nice easy pattern - easily adjusted)
B4571 (good pattern - sleeves give the impression of multilatering - not quite authentic but good look)
B4827 (sleeves would need adjusting but otherwise good 2 layer pattern)

McCall: M4805 (good mans hat pattern)
M4806 (good women's headwear - little late though)
M4490 (good standard dress - would need under-sleeves putting in and shape of sleeves slightly wrong)
M4491 (good standard dress in long sleeve version - would need under-sleeves putting in and shape of sleeves slightly wrong)

Simplicity: 9891 (ok - nothing special historically - easy pattern)
4059 (with a bit of adjustment the longer top could work)

The following is from The Evolution of Fashion - Pattern and Cut from 1066 - 1930 by Margot Hamilton Hill and Peter A Bucknell
For a picture of the way the dress looks see here

Fabrics: Large range of materials including a small quantity of brocades and silks. Garments are lined throughout in fur made up from the small pelts of short haired animals or in a material of contrasting colour or tone.
Colours: Heraldic. Please see here for a good guide.
Decoration: All-over patterns of a heraldic nature or delicate scroll repeats based on foliated plant forms. Parti-colour or quartering of costume (including the hosen). Formal dagging of the skirts of the cotehardie and the shoulder cape of the hood. Buttons as a method of fastening.
Padding and restriction - men: Cotehardie cut double with the chest parts padded to give the wearer a pigeon-chested appearance.
Padding and restriction - women: Clothes are cut and fastened (buttoning and lacing) to fit the body tightly.
Movement - men: With the figure totally revealed by the tight cutting of all garments, and the short length of the cotehardie, emphasis is placed on elegant posture, both in the stance and the holding of the arms up and and well away from the body.
Movement - women: A walk with the pelvis gently thrust forward, straight backed, with arms bent and the elbows carried away from the body to display the tippets or pendant sleeves. To prevent the great weight of the mantle slipping backwards, the fastening cords are usually held, by one hand, pulled well down on the chest.

Men:
General Characteristics: A youthful, simple, but somewhat effeminate style. Garments are cut to reveal the figure - chest, waist, hips, thighs, etc.
Shirt: Rarely seen.
Under Tunic: Dispensed with.
Cotehardie: Cut double, tightly fitting to emphasise a small waist, the shaped body parts cut in four sections. The front is buttoned through to the hem, the neckline low and boat-shaped. Tight, inserted sleeves button through from elbow point to cover most of the hand. Arm bands, with long white pendulant streamers (tippets), fasten about the upper arm.
Mantle: Three-quarters, or full circular with an opening and placket fromed to take the head. Varying in length from buttocks to ankle.
Hosen: Improvement in cut, construction and fit (material probably cut on the cross).
Shoes: Elegantly pointed. Cut away at the sides and front leaving a high back and narrow ankle strap.
Head dress: Hoods with very long liripipes and shoulder capes - possibly darted at the neck to improve fit. Narrow fillets.
Hair: Centrally parted, loosely waved, terminating at the nape of the neck. Young men sport light moustaches and small pointed beards.
Accessories: Wide, richly decorated metal hip belts, daggers, purses.

Women:
General Characteristics: Very simple, neat, youthful, and lyric in style. The under robe and cotehardie skirts are very strongly flared, the body parts laced to fit tightly and reveal the figure.
Chemise: Never seen.
Under robe: Areas visible (buttoned sleeve and hem of skirts when cotehardie skirts are lifted) made up in richly woven or embroidered cloth, the remaining parts of the robe in some simpler and less expensive material. The four sections of the body parts are strongly flared from the hips, particularly on the side seams. Laced or buttoned opening at C.F. or C.B.
Cotehardie: (lined throughout) similar in cut to the under robe. The skirts are more heavily flared and usually longer the wearer. The front opening is laced or buttoned, the neckline low and boat-shaped. Sleeves terminate at the elbow where they develop long pendulant streamers (tippets). The two slits, forming openings in the front skirts, might either give access to the belt and accessories, or be used to aid the lifting of skirts when walking.
Mantle: Three-quarter circle, with large scoop cut to take the shape of the neck, and fastened across the chest by cords.
Hosen: Only seen when the skirts are held high. Bright colours.
Shoes: Similar to the men's.
Head dress: The hair is centrally parted and often placed in cauls. Married and older women wear the veil, and barbe or wimple. Young women wear their hair very long and loose, falling about their shoulders or braided and neatly pinned and looped up, echoing the silhouette of the men's style.
Accessories: Purse, rosary, etc.

Patterns: Men   Women

Notes on patterns:
The patterns are:
- Positioned in relation to the edges of the page wherever possible to establish grain of fabric.
- Cut without allowance for seams.
- Cut in most cases without giving front or placket overlaps, overlaps to buttoned fastenings, under flaps to vents, etc.
- Basic measurements:
   Men
      Chest - 38-40 inches
      Waist - 30-32 inches
      Height - 5 feet 10 inches - 6 feet
   Women
      Bust - 35-36 inches
      Waist - 25-36 inches
      Height - 5 feet 5 inches - 5 feet 7 inches

Men
Cotehardie: Cut double. Pad between surfaces of fabric to form the pigeon chest. Add a buttoned placket to the C.F. and from the elbow points down to the sleeves.
Hosen: Use thick wollen tights a size larger than the wearer.
Mantle: Line throughout.
Hood: Line throughout. For better fit, dart at intervals around the neckline and form a buttoned C.F. opening.
Belt: Construct of metal ornaments, either sewn to a foundation, hinged, threaded together on two or more cords.

Women
Under robe: Use cotehardie pattern. Cut skirts with less flare and without train. Add buttoned plackets, from the elbow points down, to the sleeves.
Cotehardie: Line throughout. Cut skirts at least nine 9 longer than the wearer. Work eyelet holes down the C.F. edges if lacing is used as a method of fastening.
Mantle: Line throughout.