As a LIPIcs/OASIcs author, you will be invited by e-mail to register at the Dagstuhl Submission Server. The server will guide you through the publication workflow. Preliminary information can be found here:
Since the beginning of 2021, a "pdfa" option has been available in Dagstuhl Publishing's LaTeX styles, which ensures that the PDF output is "PDF/A-ready" and thus suitable for long-term archiving.