You fund your HSA-Compatible FSA through your employer. During your company's Open Enrollment period, you tell your employer how much you would like to contribute to your account for the coming year. The maximum amount you can contribute is determined by the IRS. For 2015, it is $2,550. Your employer then deducts your contribution amount (in equal portions) from your paychecks throughout the plan year.
Good news! You don't have to wait for funds to build up in your HSA-Compatible FSA. Your entire annual election amount is available to you on the first day of your plan year.