1. Go to Sql Server Management Studio >
2. Object Explorer >
3. Databases >
4. Choose and expand your Database.
5. Under your database right click on "Database Diagrams" and select "New Database Diagram".
6. It will a open a new window. Choose tables to include in ER-Diagram (to select multiple tables press "ctrl" or "shift" button and select tables).
7. Click add.
8. Wait for it to complete. Done!!

You can save generated diagram for future use.