How do I file a 1099-K which paid me for umpiring/refereeing?

As self-employment income.  Complete a Schedule C in TurboTax.  After you complete the profile (you can use your name and home address for the business), you will be ask to enter your self-employment income.   Enter the amount from the 1099-K.   And then you will be prompted to deduct your business related expenses.   You can deduct ordinary and necessary expenses as well as take the QBI deduction for these earnings.   

Where do I enter Schedule C?   

How do I report income from self-employment?  you will use the Income from cash, personal checks, credit card payments, or cryptocurrency (Form 1099-K) related to your self-employment entry spot.  

What self-employed expenses can I deduct?  **See below - these and other common expenses available to all self-employed taxpayers are listed.

Where do I enter my self-employment business expenses, like supplies and  vehicle mileage?

**As a self-employed coach, umpire, or referee, here are some additional expenses you can deduct:

• Tools, supplies, and equipment (whistles, balls, cones, first aid kit, clipboard, etc.)
• Uniforms or gear not suitable for everyday wear, such as helmets, masks, skates, and chest protectors
• CPR certification and first aid training
• Coach’s clinics, workshops, and specialized training
• Travel expenses such as airfare, auto rental, and lodging (plus meals, if you have to be away overnight)
• Business portion of mobile phones, data plans, or a second phone line
• Vehicle expenses or standard mileage
• Advertising, including websites
• Licenses, permits, fees, and membership dues