Step 1: Select “Create Record” Node in Journey Facilitator canvas. 

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Step 2: Set the following details as shown in the image below. Change the From Social Network field depending on the channel on which you will be sending the message later on (Say Email, SMS, etc.). From Social Network User ID should always be return PROFILE.SN_ID.get(0);

 
Apart from the 2 fields you just set, you can also add the case custom fields you want to set on your case here. 
Now,you can give a variable name of choice that can be used to call this case later. 

Step 3: Set Universal case in context of the journey using the “variable operation” node and following 2 actions 
UNIVERSAL_CASE Set newCase.ASSET_ID 

CASE_NUMBER Set newCase.CASE_NUMBER 

Note: newCase here is the case variable created in Step 2 

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Step4: Now you can send messages/Surveys using Send Message or Send Resolved Message or Send Survey node and they will automatically attach to the case created.