How do I declare money from a sublet?

Yes, you must declare the income you received from subletting your apartment.  You will need to treat this as Rental Income and report it on a Schedule E.

For more information, please see Djay 43's answer below: 

https://ttlc.intuit.com/replies/4284787

if not in business of renting property and sublet for same or less than rent due, aren't expenses only deductible as itemized deductions on schedule A and subject to 2% floor now?

Please stick to your other post.

"You can deduct the rent you pay to your landlord because it is an ordinary and necessary expense for you to rent the property". Is this still valid if one doesn't have rental income for the sublet in the period one pays the rent to the landlord?

From where are you quoting?

From Djay 43's answer quoted above in TurboTaxChelsiE , CPA's answer