IRS Tax Tip 2018-112 gives advice for claiming the home office deduction:
This method requires computing the business use of the home by dividing the expenses of operating the home between personal and business use. Direct business expenses are fully deductible and the percentage of the home floor space used for business is assignable to indirect total expenses.
The simplified method has a prescribed rate of $5 a square foot for business use of the home. There is a maximum allowable deduction available based on up to 300 square feet. Choosing this option requires taxpayers to complete a short worksheet in the tax instructions and enter the result on the tax return. There is a special calculation for daycare providers.
Regardless of the method used to compute the deduction, business expenses in excess of the gross income limitation are not deductible. Deductible expenses for business use of a home include the business portion of real estate taxes, mortgage interest, rent, casualty losses, utilities, insurance, depreciation, maintenance and repairs. In general, expenses for the parts of the home not used for business are not deductible.
Deductions for business storage are allowed when the home is the only fixed location of the business, or for regular use of a residence for daycare services; exclusive use isn’t required in these cases.
IRS Publication 587gives more information about both methods.