To design a many-to-many relationship between two tables, create a junction table to link them together.
Create the junction table using the primary key from each table.
Example
Table_A has a primary key column named Column_A.
Table_B has a primary key column named Column_B.
Figure 1. Initial tables, each with a primary key defined
To create the many-to-many relationship, add a junction table. This table is comprised of the primary key columns from each of the other tables.
Figure 2. Create and include the junction table