Philadelphia BIRT and NPT (originally tried to post in ELI5) Need Simple Explination

I'm self employed/freelance, non-resident of Philadelphia, and a small portion of my income is in the city of Philadelphia. Are there easier to understand instructions for Philly's BIRT (Business Income & Receipts Tax) and NPT (Net Profits Tax)? The forms and instructions I've found are extremely difficult to understand for those of us not schooled in tax law verbiage. IRS forms are easier to understand. Thanks!!