The one Setup tool that shows your data model as it is, not as the ERD on the wiki claims it is.
Read before you draw
Open Schema Builder on any org you inherit. The orphaned objects and mystery lookups you find in ten minutes would take a week of clicking through Object Manager.
Draw carefully
Schema Builder can create fields and relationships directly — convenient in a Developer Edition, a footgun in production. Look with it; build through your normal deployment path.
Practice in your org
- Open your Developer Edition org and follow along step by step
- Change one thing at a time and note what happens
- When something breaks, read the error before searching for it