Working alone requires a lot of imagination. In this case I had a unit 94” long (and fairly heavy) to mount to a masonry wall.
I decided to add a 1×4 oak slat under the center section of the main shelf that I would mount to the wall first. I drilled holes in the top of the slat and inserted 4 pieces of 1/4” steel rod that would protrude 3/4”.
These mate with holes in the main shelf and end up flush with the top surface. I countersunk the underside of the holes in the shelf and drilled them 5/16” rather than 1/4” so I’d have a prayer of mating the holes and rods, on a ladder holding a unit eight feet long.
This worked surprisingly well. I added two 2×3s to support the shelf at the front, just in case.
With the unit secure, I could drill the pilot holes for the masonry anchors. The unit alone must weigh 35 pounds and it will have a fair amount of mostly ceramic stuff on it. I ended up using 9 anchors total.