That's very simple. If going by car, just follow highway R10 (direction Mladá Boleslav, Turnov, Liberec) and take exit 33 for road 275 (direction Luštěnice, Jabkenice). In the end of village Rejšice, turn right (direction Chudíř, Loučeň). You will see castle parking place just in the begining of the village Loučeň. For the route planning, see Mapy CZ web portal. Loučeň is also accessible by public bus. You have to switch buses in Mladá Boleslav or Nymburk (there are frequent bus and train connections to both these towns from Prague). You can find ideal public transport connections using website IDOS CZ.