With difficulty, and sometimes not at all.

Artillery barrages would be elaborately planned before a major offensive: the front would be heavily scouted from observation posts in No Man's Land (for example, the famous hollow fake trees), by night patrols raiding into the enemy's trenches, and by aircraft. Control of the air was critical to offensive success: at the Battle of Amiens in 1918, British aircraft identified 504 of the Germans' 530 guns and allowed them to be neutralised before the attack, immensely weakening the Germans' ability to resist. By 1917, the British Army had refined the hurricane bombardment (an incredibly heavy, but short barrage to destroy and stun the enemy immediately before the offensive was launched), as well as the creeping barrage (where the barrage moved slowly forward ahead to the advancing troops to a set timetable) to a high art.

However, all of this required extreme planning and preparation. The nature of First World War communication was that commanders abandoned the ability to talk to higher headquarters when they left the front line trenches. Communications were dependent on buried telephone wire or runners. Radios were in their infancy: the British Army's best trench radio required twelve men to carry, largely because of the bulk and weight of its batteries. There are many instances of forward observation officers observing and correcting the fall of fire prior to an offensive, but this required the pre-laying and burying of telephone wire. For the great majority of officers leading attacks, there was never an ability to "call in the fire mission!"

Sources:

Gordon Corrigan, Mud, Blood and Poppycock

John Keegan, The First World War

Simkins, Jukes and Hickey, The First World War: The War to End All Wars