Navigate to your Job > Job Financials > Variations and open the relevant variation or click 
to create.
Use the Link to selection set drop-down to select your desired selection set. Ensure the set has no entered costs, as any approved sets with allocated costs differing from the allowance or budget will automatically create a variation.
Once the variation is linked to a selection set, it cannot be approved or declined until the status of the linked variation is either approved or declined.
For more information on how variations are automatically created with a new selection set, see this article.