All applicants for FERS Disability Retirement must fill out an Application for Immediate Retirement (SF3107) and the Documentation in Support of Disability Retirement Application (SF3112). The "Documentation" is a number of separate forms, including the Applicant's Statement (3112A), the Supervisor's Statement (3112B) and supporting documents, the Physician's Statement (3112C), and the Agency Certification of Reassignment and Accommodation Efforts (3112D).
All applicants must provide OPM with proof that they have applied for Social Security Disability Insurance (SSDI) before OPM will begin processing their application. You can apply online.
However, just filling out these forms is not always enough. They must be filled out in a way that convinces OPM of your right to Disability Retirement. This is where a lawyer comes into play.
The three most important documents you can control are the Applicant's Statement, Physician's Statement, and Supervisor's Statement. Read about the Supervisor's Statement here and the Applicant's Statement and Physician's Statement here.