It is described in the statute as a "petition." It must be in writing but can be in letter form. It must attach a copy of the original request to custodian and any written response from custodian. 950 CMR 32.08(2).
A fairly detailed description should have been included in the original request to custodian.
The letter should include a brief statement as to why record is public or, if custodian has given reason for denial, refutation of that reason.
The Law requires that the appeal “state the grounds upon which the requester asserts that the record is a public record, legislative record or financial record and shall address any grounds stated by the agency for delaying or denying the request.” 65 Pa. C.S.A. § 67.1101(a)(1). The Office of Open Records maintains a standard appeal form on its website. https://www.openrecords.pa.gov/Appeals/ProceduralGuidelines.cfm.
An appeal must identify the record requested. The Law requires that the appeal “address any grounds stated by the agency for delaying or denying the request.” Id. § 67.1101(a)(1); see, e.g., Dep’t of Corr. v. Office of Open Records, 18 A.3d 429 (Pa. Commw. Ct. 2011) (reversing Office of Open Records determination that the requester sufficiently addressed the grounds for denial by articulating the procedural history of the request and stating that the “right to know requests are public”).
There are no statutory requirements for the appeal to the agency head; it is not even required that it be in writing, although in practice it should be to be of any real use. The appeal need not explain why the denial is erroneous, but doing so would probably be helpful. At the very least, the appeal should attach or refer to the written denial of the custodian and state that it is being appealed.