I assume they weren't able to sit in front of a computer for the whole day like we do today. So how did they write their program? On a piece of paper and type it later when the computer is available? How did they do their testing?
How did programmers work back when a computer was very expensive, rare, as big as a room?
history
Circa 1974, you'd sit at a convenient desk and write your program out long hand on paper. You'd test it by walking through it in your head using test data. When you were satisfied that your program was correct, you'd go to the punch card room and transcribe your program onto punch cards, one 80 character line per card. You'd also punch cards for any data your program might need. Then you'd also punch a few incredibly cryptic cards in Job Control Language (JCL) that would tell the computer how to compile and run your program, and what input/output devices it would use. Then you'd take your cards to the 'IO Window', where you'd hand them to a clerk. When your turn came, the clerk would load your cards into a hopper, and push a button to tell the computer to start reading them. The output of your program would generally go to a line printer or a drum plotter. When your program was done, the clerk would collect your cards, and your hard copy output, and put them in a pigeon hole where you could pick them up. You'd pick up the  output, review the resuilts, and repeat the process. It would take anywhere from 20 minutes to 24 hours for a complete cycle. You can probably imagine that you were not happy when you found that the only output was a printed message from the compiler telling you that your program had a syntax error.You might also have access to a computer through a teletype, so you could actually have an interactive session with a remote computer. However, typing on a teletype was physically painful (very stiff keys, and loud), so you still generally wrote and tested your program on paper first.By 1976 UNIX systems and mini-computers like the PDP 11-70 were becoming more common. You usually worked in a room full of video terminals with 25x80 character displays. These were connected to the computer via serial lines. Crude, but not too dissimilar from working at a command prompt today. Most editors back then were pretty crappy though. Vi was an amazing improvement.