If you have already e-filed and are just sending in a payment, enclose the check with the Pennsylvania payment voucher. This voucher is populated within your tax return and you can access it by printing your tax forms. Alternatively, you can obtain the voucher directly from the Pennsylvania Department of Revenue website: PA-40 PAYMENT VOUCHER.
If you have not e-filed and have a payment to make with the return, print the entire return, voucher, and mail with the check.
See: How do I get a PDF of my return?