You'll find the Snippet feature in the Assets dashboard of your account. Navigate to Assets, then click 'Snippets' from the left navigation.
1. Click the 'Create Snippet' button, top right.
2. Give the Snippet a recognizable name, then click Save and Next.
3. Choose how to start the design: Blank, Paste HTML, Upload HTML, Template.
4. Proceed in designing the Snippet, then Save and Exit. Your new Snippet will be saved to the library in your account.
5. Hover over new Snippet in the library and locate the Edit drop-down menu. Select 'Complete'.
Snippets must be in the complete status to insert them into an email template or campaign.