Beyond the exact-match phrase you're targeting, related terms play a supporting role across nearly every on-page element of a well-optimized page.
Weave Them Naturally Into Body Content
The most natural place for related terms is throughout the actual explanation and discussion in your body copy, not forced into a separate section.
Use Them to Vary Subheadings
Subheadings built around related subtopics, rather than repeating the same phrase each time, both read better and cover more ground.
Don't Force Them Where They Don't Fit
A related term that doesn't fit naturally into a sentence is better left out than awkwardly inserted just to check a box.
Next step: Use the LSI Tool to gather related terms worth incorporating across your page's on-page elements.