In the contract, you add a document for signature.
You can make it signable on the portal (we use the Partner Portal for that) or make it not available on the portal, meaning it has to be signed in the contract.
To sign the contract in IMPOWR, after saving, when the signer accesses the contract in edit mode, they go to the 'Documents for Signature' tab. They click on the 'View Signatures' eye and can see that they document is not signed. There is a 'Sign' button there - click on it and enter the necessary items to verify your signature.
There are some documents in the Knowledgebase which address the signing of contracts both in the portal and not in the portal:
In the Portal: How do I sign unsign a contract document
Not in the Portal: How do I prepare a contract