There is a missing photo on Page 46 of the 'Expand your Sten-v5' notes describing attachment to existing wall studs.  Here is a copy:

Notes:

•  The horizontal strip is a 1 x 2 length of softwood lumber secured to the wall studs behind the shelf with two 1½ inch No. 8 flat-head steel wood screws at each stud.  

• Drill the 1x2 for a 5/16in bolt through an existing unused hole in a Sten upright. In the photo, the bolt head is visible in the Sten Upright.

• Secure the 5/16in bolt with washers on both sides and hex nut.