Start by deciding who you are writing to - your girlfriend, your family, or someone else. This may make a difference in how you open or close the letter or how much detail you include.
You might start by explaining the setting of the battle, describing the trenches and barbed wire and the distance between the German and Canadian lines before the battle started. This was an area that was very strongly defended by the Germans, so there would be lots to talk about. You will also need to mention the weather, probably to comment on how difficult it was to see, move, or understand exactly what was happening in the blizzard.
You can decide exactly what your responsibilities were during the battle. Were you a soldier trying to crawl through the snow and slush and mud to get across no-man's land? Were you a medic, dashing from hole to hole to try to care for the wounded? Use your imagination - think about the war movies you've seen and put yourself in the middle of the action.
To end your letter, you might report on friends who were killed or injured in the battle after reassuring the letter's reader that you are alright (or that your injuries are minor, or whatever you decide). You might comment on the overall number of casualties, the amount of land now controlled by the Canadian troops, and how you felt after the battle had ended (tired, cold, wet, hungry?). Send your love or best thoughts along with the letter, ask for news from home, and tell them you hope to see them soon!