He studied at the State School of Applied Arts with Prof. Jaroslav Benda (1941–43) and Prof. Antonin Strnadel (1945–47). During the intervening war years he worked in the chemigraphical department of the State Printing House. After graduation Flejšar co-operated with a group of promotional graphic designers to form Tvar (Shape), the association of visual artists. Between 1956 and 1959 he and Josef Raban provided the creative leadership of the Promotion Cooperative. He was a co-founder of Bilance, a group of visual artists. He participated in design work for the Czechoslovak Chamber of Commerce in Vienna, Helsinki, Leipzig, Kiev, Damascus, Paris, Brussels, Zagreb, Moscow, Poznan and Budapest. He designed the Czechoslovak exhibitions for the Brussels and Montreal Expos and the International Trade Fair in Düsseldorf 1975. He co-operated with the foreign trade organization Jablonex on exhibition designs for Montreal (1969) and Leningrad (1970).