As with the other ligand puzzles, the first step is to find the ligand and click on it to select it.
Then find the reaction design tool in the action bar. It should appear on the right, and it doesn't have a keyboard shortcut.
Click on the reaction design icon to open the reaction design window.
The reaction design window shows two "reaction types" across the top. Below the reaction types, there are two columns of "reagents".
The goal is to select a reaction type, then one reagent from each column. (A very subtle gray border appears around the selected choices.)
Once you've made your selections, click "Accept Reaction", and then wiggle to finish. If your ligand fails to score, reset the puzzle (Ctrl+R) and try again.