So you're trying to recreate a recruitment poster correct? I agree that you need to do some research on the subject and how they recruited volunteers and troops for WWI. Poor souls didn't know what they were letting themselves in for then as it was very brutal. My grandfather and brother were in WWI, my great uncle died in a famous battle involving Newfoundlanders. Pure tragedy.
However, your best bet for research would be the federal government's V
eteran's Affairs. They have a lot of historic documents and information that should give you some ideas.
I would imagine there were themes of patriotism as well as training/job opportunities, the same as there are today, around recruiting people to join up,. Your recruitment poster would need to reflect the attitude of the day to make it authentic.
Here are some posters that were used in WW! in Canada.