Step 1. Create a custom dimension in GA with Name and Scope and hit Create.
For example: In GA - Custom Dimension's Name - Store_Hits
Scope - Hits
After hitting create you get an index number for the GA custom dimension. For this example Index - 1
Step 2: Then go to your Tealium account. Create a data layer (best practice will be to keep the name of the data layer UDO variable same as GA custom dimension you just created in GA)
Step 3: Then go to Google Universal Analytics Tag (hopefully you have added Google Universal Analytics tag type) in your Tealium Account.
Step 4: Go to Data Mappings in the tag settings and find the data layer you just created with help of 'Select Destination'.
Step 5: Hit 'Add Destination' - Go to mapped tool box dialogue box - click dimensions - select event type as Pageviews - click Apply - the pageview-dimension1 will appear for Data Layer Store_Hits and again click Apply.
You have successfully created GA's custom dimension in Tealium.