Step 1
Before you can add a signature to emails, you need to create it under the Senders menu. Check this tutorial to learn how you create a signature for each sender.
Step 2
Once you created a signature for your sender, you can add it to your email by clicking the Personalize menu above the email text editor and selecting SENDER SIGNATURE which looks like {{SENDER_SIGNATURE}}
Under Sender info, you can also find other Sender-related variables, that you can include anywhere in the email copy.
You need to add {{SENDER_SIGNATURE}} to every email where you want it to appear. Adding Sender-related variables gives you the flexibility to mix email structure. For example, the initial email has a signature, while the follow-up only has the sender's name.