1. Unpack the uprights. Position them with their two holes over the two holes in the back of the workbench. Drop the two long screws into the aligned holes. Put washers and nuts on the bottom and tighten.
2. Each upright has two exclusive vertical adjustment screws on the bottom. These are rarely used. After you load-up your shelves, see if the uprights are still perfectly vertical. If not, tighten these screws as needed.