Satisfy the requirements (be a member of a club and have them act as one of your referees). Put in for your FAC for your required firearm.
EG: .44 muzzle Loading revolver
Buy firearm,
Use black-powder equivalents like 777 or pyrodex, then no Explosive licence or HSE RCA document required.
If you decide that you want to use genuine Black Powder explosive, then you will require an Explosives licence and a HSE RCA (Recipient Competent Authority) to transport the black powder.
http://www.hse.gov.uk/explosives/information/rca.htm