Air goes through the pre-cooler (a series of heat-exchangers which cool it down from over 1000C to minus 150C) after which the air goes through two compressors and is mixed with the fuel and helium in the pre-burner.
Alongside this a hot air stream is mixed with hot gas (from liquid hydrogen) in the ram-jet burners.
Oxidizer and hydrogen closed cycle:
Liquid Oxygen and Liquid Hydrogen are mixed in pre-burners and hot gases are pumped through the main gas manifolds to the combustion chamber.
Hydrogen is injected directly into the main combustion chamber where magnetic coils compress and ionize the hydrogen to create a plasma that is ejected through a magnetized nozzle.
Note that radiation shields are necessary to protect the ship and crew from radiation. For this reason Hydrogen fusion is not used while in-atmosphere and near space stations/other ships.