Trimming and shaping bales
Before plastering it is advisable and very satisfying to trim the bales. Removing any loose straws, 'steps' between neighbouring bales and ridges between folded 'packs' in the bales will all make the plastering process easier and more economical as well as giving a smoother end result.
I have always used a chainsaw for this job. A hedge trimmer would probably work better and garden shears will do the job albeit a bit slower. I have also spoken to people who swear by an angle grinder with metal cutting disc.
Whilst you're doing this job you can start to get quite sculptural. For example, at windows you can make quite a significant curve (reveal) as in the photo below. Just be careful not to cut the strings!
Stuffed Hessian Shapes
In the photo to the right, the sides of the window were curved off as described above. The plastered form over the window was made using the following method which seemed to work particularly well.
Hessian sacking was stapled, fairly tightly, between the window frame and roof timbers. This was done all across the top of the window on the outside with the top and bottom edges tucked inside. Then from the inside I did the same stapling a foot or so of sack on at a time and stuffing straw in the gap.
When stuffing remove straw carefully from the bale to keep it as tightly packed as possible. At the ends where the form meets the wall simply overlap the hessian by 6″ (15cm) or so then plaster over the whole lot.