How to create junction object?

A custom object with two master-detail relationships. Using a custom junction object, you can model a “many-to-many” relationship between two objects.

Step 1. Create Object

Create > Objects > New Custom Object

Do not launch Custom Tab Wizard after saving this custom object.

Object created

Create two master detail relationship

Custom Fields & Relationships > New

Select Master Detail Relationship

Related To > Account (Choose the related object)

Enter Field Name

Sharing Settings: Sharing access that users must have to a master record in order to create, edit, or delete its associated detail records.

Next

Field-Level Security for reference field (Cannot be changed).

Next

Add reference field to Page Layouts > Next

Add custom related list > Save (Master-detail relationship field is created)

Similarly create second master-detail relationship field on the object.

Account Object > Select Record (Related list will be created on the record)