1:1
Combine the tables or take the primary key from one into the other.
1:1 with Optionality on one end
Take the primary key from the non-optional end and put it as a foreign key in the optional end.
1:1 with Optionality on both ends
Create a new link table using the primary key from one table as a foreign primary key in the new table. The other table's primary key is also put in the new table as a foreign key.
1:M
1:M with Optionality on the many end
Primary key from the 1 side of the relationship is put into the many table as a foreign key.
1:M with Optionality on the one end
1:M with Optionality on both ends
Create a new link table using the primary key from the one end as a foreign primary key in the new table. The primary key from the many end is also put in the new table as a foreign key.
M:N
M:N with Optionality on one end
M:N with Optionality on both ends
Create a new link table using the primary keys of each table as a compound primary key in the new table