You ultimately sufficient proof of income, genuine relationship and accommodation.
https://www.gov.uk/government/uploads/s ... lement.pdf
This link provides general information on what you need to support your application.
https://www.gov.uk/guidance/immigration ... d-evidence
This link provides specific details on information you need to submit for proof of finance/earnings to sponsor your partner.
If you also search spouse visa posts on here it will give you an idea of what you need to submit...
If you are paying for the priority service I believe you would submit at the embassy rather than sending to Sheffield.