If your employer is reimbursing you for your expenses, you only need to prove them to him/her. To do this, submit a written accounting to the employer and return any excess amounts.
“Accountable plans” or per diem arrangements and mileage allowances are used instead of detailed accounting for the employer, if place, time, and business purpose are verified.
Detailed documentation is required by the IRS when expenses aren’t fully reimbursed by the employer or you fail or are not required to return excess reimbursements. If you are the employee, a 2% of AGI floor on miscellaneous itemized deductions is applied to your deductions. However, these deductions are not allowed to an employee from 2018-2025 unless you are an Armed Forces reservist, qualified performing artist, fee-basis state or local government officials, or employee with impairment-related work expenses. If you meet one of these categories, your expenses are deductible on Schedule 1 of Form 1040 and are not subject to the 2% of AGI floor on miscellaneous itemized deductions.
You should record the expenses as close to the time of expenditure as possible.