There are several very interesting tutorials and resources online, so I won't be writing one myself :-). The first place to start when you're interested in aprons is Larsdatter's picture archive:http://www.larsdatter.com/aprons.htm
Tutorials can be found here:
Catrijn, Louisa and Matilda, thanks for the tutorials! In each of the tutorials, honeycomb stitch was used. I was told there are two ways of making these: a) fold the pleads first and then stitch on top of the pleads, or b) make the pleads while stitching at the same time. I used method A (see picture above), click here and scroll down:
A tutorial of method B can be found here:
A technical note based on my experience so far: My needlecraft encyclopedia recommended using 3 times as much fabric as the required length of the finished piece. This means that an apron of 50 cm would require 150 cm of fabric. However, honeycomb stitch is very stretchy, so if you use only honeycomb stitch, you might need only twice or 2,5 times as much fabric. The other smock stitches are not quite so flexible, so if you plan to combine honeycomb with other stitches, it is probably a good idea to use more fabric. It's really important to take the time to figure this out, because I've seen some reproductions of smocked aprons which were too small, and that's a pity for all the hard work!
The aprons from the Lutrell Psalter (see Larsdatter above) seem to be embroidered with different stitch types and probably require a rather extensive length of fabric. This makes me wonder: using 150 cm of linen fabric for an apron seems quite “decadent” for such a basic, working class item, especially when you can also make a plain apron using only 50 cm. Where these types of smocked aprons really part of the working class wardrobe or were they a nice, decorative addition to the wardrobe of more afluent social groups? I don't know, but it would be interesting to read more about that...