You need to contact the firm that sent you the letter, fill out the one page form, attach W2s, and mail it to the firm.  PA local taxes collections are usually contracted out to an agent such as Keystone or Berkheimer.  The rates in PA range from 0.5% to 3% depending on where you reside.

The local earned income tax applies to W2 wages and self-employment income.   If you have self-employment income, attach a copy of the PA state income tax Schedule C.