I found these to be excellent items - with slatted shelves you can be flexible about the shelf width - which means you can have the shelf fit tight from the frame to the nut at the end of the bracket.
I first fitted the brackets to the frame (ensuring they were level), Then, for my slatted shelves to fit, I first added the cross pieces of wood at each end (screwed into the slats) to achieve the desired shelf width from frame to nut inside bracket end (I measured this to be 26.5cm).
I then temporarily fitted the shelves onto the brackets and drew a line across the slats either side of the brackets (underneath) before removing the shelves and adding a few more cross pieces of wood screwed underneath the slats either side of the brackets to help brace the slats and secure the shelves snugly to fit the brackets, before replacing. these shelves are now not moving!
The 10 inch shelves comfortably hold seed/gravel trays, and I have them holding shuttle trays of 18 (6 x 3) x 8cm pots from Suttons. The 6 inch shelves can also just about hold seed trays without fear of toppling, but are better for large (6inch) pots.