You can relate a Parent Contract from the About Box (A) of a Child Contract, or from the Related Contracts tab above the grid (B). To add or edit the Related Contract, click the Related Contract link, NOT the name of the Contract itself.

When Relating a Contract you are asked the following questions:

Related to is Archived? - This is asking if you're relating the current contract to one that's been moved to the 'archived' stage. The most common answer to this question is NO.
What Contract Type? - is requesting to know the contract type of the contract you wish to relate to. You can choose the contract type (if you know it) to narrow down the options presented in the last field. If you don't remember, select 'All'
Which Contract? - asks what contract specifically you're relating to this one. Choose the contract you wish to relate to and hit Save. 

To Unlink Related Contracts:

Click on the words "Related Contract" in the summary of a contract container, this will reset all drop-down lists to the following criteria:

Drop Down 1: Related To is Archived?
Drop Down 2: What Contract Type?
Drop Down 3: Which Contract?

Be sure to hit Save. This will disassociate the contract.