You dont have to sign on - like when claiming JA/JB. You just have to go to the clinic and make the original application to the CWO. Once the claim is processed - which involves a means test you are then paid weekly by giro. I'd bring identification, P60 (if applicable), bank a/c or details of savings (if applicable), details of all outgoings and expenses (rent/mortgage, esb etc), medical expenses (if applicable)... any information that will help your case.
Since you made the application for OFP - you may also be entitled to secondary entitlements like fuel allowance, esb, telephone etc and rent allowance. The local Citizens Information Service would have application forms. I dont know whether the latter is available online from the DSFA - but you can check it out. If you dont already have a medical card - you may be entitled to the full GMS card or GP only visit card - again depending on your means. This application form is got from Citizens Information Services.