This is your under-garment, it is generally calf length with full length sleeves with a fitted cuff. The shift should be full enough that it doesn't restrict your movement. Most shifts are only just visible at the neck of dresses and as pictures of commoners tend to be small such detail is rarely visible. However, on those pictures where detail is visible, which are often higher up the social scale, neck lines seem either to be of a fitted nature or pleated into a fixed band, rather than gathered. Therefore, unless new evidence comes to light, new shifts should not have drawstrings at the neck or wrist.
There are two versions of a shift, both tend to be only slightly visible. The first version is a low necked shirt, this should either have a neckline roughly following your dress neckline, or it could be V-necked resulting in two wedge shaped pieces visible either side of the neck, see pictures. The V-necked type only appears in the later part of the 15th century and is generally worn with a square necked dress; so don't wear it if the date of the event is before about 1475-80, and then only if you are of moderate station. Either type can be made like a man's shirt but longer, also don't make the sleeves too baggy unless your material is fine as they have to fit under fairly tight dress sleeves.
The other version is to make a pleated shift. This is made from very large rectangles which are pleated into a band at the neck and wrist. The sleeve piece goes right up to the neck band and therefore must be longer to accommodate this. The band must either be cut on the bias to allow it to be flat, or the shift must be square necked with a seam at the comers. This style should not come much higher than the dress neckline as a general rule.
Material: Linen, about 2m, depending on your need and design.
Colours : White or off white depending on your station