18th Century Shift:

Neckline options
-- gathered to a narrow band (most common in the mid-18th in depictions of working class people in France);
-- cut to the exact dimensions of the finished neckline and not gathered at all, but simply finished off with a rolled edge;
-- sewn to a casing, larger than the finished neckline, through which a drawstring is run, then gathered;
-- finished with a ruffle (more common the higher in the social scale you go) -- not wider than 1-1/2" to 2".  Lace would not be worn unless you are portraying a lady.

Sleeve options
-- gathered to a narrow (1/2" to 3/4") cuff, which is then possibly either buttoned like a man's shirt, with a single thread button and buttonhole, or two buttonholes and linked buttons (like cufflinks), or a tie passed through the two eyelets on either end of the cuff and tied together (most common in mid-15th c. paintings);
-- not gathered at all, but finished off with a rolled edge (better for narrow sleeves);
-- sewn to a casing, through which a drawstring is passed, then gathered;
-- finished with a ruffle (again, more common higher in the social scale).

Sleeve length
-- Usually below the elbow, leaving enough ease for the arm to bend easily when the cuff is fastened;
-- Very occasionally wrist-length -- but I have not yet seen a depiction of a long shift sleeve worn by a young person.  All three women wearing long shift sleeves were a) middle-aged to elderly, and b) wearing long jacket sleeves.

Shift length should be at least knee-length, usually mid-calf, unless you're planning on wearing a 'cutty' (short) 'sark' (shift), like the witch in Burns's poem, in which case mid-thigh might work!

Materials:
Linen is the best choice, since it breathes better than cotton or cotton-poly blends, but because of the expense of linen, it's a good idea to make your 'first draft' shift out of cotton muslin, which is fairly cheap.  Buy 100% cotton, not a cotton-poly blend, if you'll be out in hot weather; polyester is basically plastic and doesn't breathe well. 
Cotton was very expensive in the 18th century -- more expensive than linen -- so poor women would not have been likely to own a cotton shift.  In addition to breathing better, linen also takes more abuse -- it's a much stronger cloth than cotton muslin.  So, once you get a shift pattern that fits well, it's a good idea to make a linen shift.