Financial & Legal
You need to be either a UK Seaman (be a UK resident) or serve on a UK flagged vessel (UK, Jersey, etc). The other Red Ensign Flags (Cayman etc) have their own and you will need to apply to them instead.
Apply to the Registry of Shipping and Seaman in Cardiff. They are a Branch of the MCA.
I simply phoned them up and asked them to post me the form.
Fill out the form, attach some passport photos, get the required signatures (or attach letters if not possible) and pay the fee.
I seem to remember it was about £35. They even took my non-UK credit card (but you need to ask specifically for this).
Otherwise they were very helpful, pleasant, and efficient (much more so than MCA Seafarer Training Branch in Southampton). I think it only took a few weeks all together.
If the following details are not correct phone the MCA in Southampton and ask for the right number:
Registry of Shipping and Seaman, MCA Cardiff, Ground Floor, Anchor Court, Keen Road, Cardiff CF24 5JW, United Kingdom.
Ph: +44 29 2044 8800
Hope this helps :)
PS: Remember that with a discharge book you can also obtain certain benefits such as increased backage allowance on air flights, one-way ticket and half the price of a return, etc. Sometimes its handy.