Hi there, this is taken from an earlier answer to a similar question:

There wasn't actually that much of France under German occupation during the war. After the breakdown of the Schlieffen plan and the race to the sea, the front line more or less stabilised, although there would be shifts as various offensives took place. You can see on this map that most of the occupied territory is on the Belgian border. This territory was mainly two thirds of the Department (a French administrative area) of Nord, which was home to roughly 1.176 million French citizens. Altogether, parts of ten Departments were occupied, with a total population of about 2 million. As many men had fled before the German advance or been mobilised into the army, a majority of the population was female.

Unfortunately for the French, this strip of land was one of the most heavily industrialised in the country, producing 60% of its iron, a quarter of its steel and up to 40% of its coal. This was very useful to the Germans, who had found themselves deprived of seaborne imports by the British blockade. Increasingly, raw materials and industrial machinery were taken to Germany to assist with the war effort. The entire area of occupied France was also close enough to the front lines that they fell under military, rather than civil, administration. There was widespread requistion of food, and civilians were increasingly forced to work for the occupiers, building fortifications and carrying out duties usually reserved for pioneer units. Those who refused to work voluntarily were forced to work in Civil-Worker Battalions, which were generally brutal, with poor rations. There were high mortality rates and the workers were interned in special camps. In summer 1916, 20,000 workers were deported to the Ardennes to work on the harvest, although this practice was stopped after widespread international protest.

In terms of administration, French authorities were mostly sidelined. Instead, the occupied zone was divided into various divisions called Etappen, under the control of an Inspector, with subdivisions under the control of the area Commandant. The army in the area at the time was responsible for providing administrators to oversee each Etappe, and when the armies were redeployed, so were their administrators. The subdivisions could vary in size, with the largest, such as the city of Lille, being administered by a high ranking officer and dozens of soldiers, whereas small villages could only have an NCO assigned to them. Each citizen was issued an identity card, and if they were found outside their Etappe then they could be faced with a fine or imprisonment. It was possible to obtain a pass from the area Commandant, but this was a difficult and complicated process, and the passes generally only lasted a few days. While a military necessity from the perspective of the Germans, this system led to increased frustration among the French population. The French police were left to deal with crimes against other French people, but the Germans had their own military and police force.

Correspondence with anyone outside the occupied zone was initially punishable by death, but the punishment soon decreased and there was in any case a thriving underground post network, often via neutral countries or the Red Cross. The German Army also produced a newspaper, called the Gazette des Ardennes, which provided reports on news from the front and extracts from British and French newspapers. As the French also had access to German language newspapers, it was important that the Gazette didn't appear as overt propaganda. As such, it was more or less factually trustworthy, but had a heavy pro-German perspective. It's difficult to tell exactly how popular it was. Whilst circulation reached 180,000 by January 1918, accounts by French citizens mention that much of the time it wasn't actually read. In an effort to counter this, the British and French airdropped newspapers and pamphlets over the occupied territories. There were also a few underground newspapers, and while owning a radio was illegal, those who had one hidden were able to listen to radio stations on the other side of the front line. The French also conversed openly with soldiers billeted in the area, allowing them to gain information in that way.

In terms of resistance, there was very little that the French could do to actively resist the occupiers. Those who refused to carry out work or were found guilty of carrying out resistance were conscripted into labour battalions. Underground newspapers and networks to help escaped prisoners were made very difficult by the tight grip of the German army, although they did exist. For example, in June 1915 a German sentry was shot by a Frenchman near Roubiax. However, several spies and saboteurs were shot by the German army in response. Many ordinary French people limited themselves to symbolic acts of resistance, such as refusing to shake hands with Germans, wearing the national colours and writing letters of protest. There is some evidence to show that letters of protest actually had a negative effect on the war effort, as area Commandants had to spend a lot of their time responding to the letters, and became increasingly irritated as a result of them. The French also refused to give the Germans lists of military age men and the civil authorities were generally obstructive towards the Germans, relying on legalistic interpretations of international law on what occupying armies were allowed to requisition. In response, the Germans would threaten the authorities with various fines or harsh punishments.

However, the close proximity in which the Germans and French lived and worked meant that contact and co-operation, if not collaboration, was commonplace. A number of French women had relationships and even children with German soldiers, and accounts of the occupation accuse a large number of both working and middle-class women of having sexual relationships with the occupiers. A report for British intelligence in 1918 listed 362 women who had relationships with Germans, ranging from prostitution to having children with them, and the number of women treated for sexually transmitted diseases rose sharply with the arrival of the Germans. Women who did not have relationships with Germans, but interacted with them socially, were often the subject of disapproval, and gossip could easily overstate the nature of their relationships. This disapproval was also extended to some men who were socially friendly with the Germans. A number of high ranking civil servants were accused of being too friendly with the Germans, although evidence that this was true is lacking. Angry mobs would occasionally form, such as in Lille where a woman was pelted with stones and called a whore by a crowd of almost 500 people. It took the German police firing above the crowd to disperse them. After the war there was little done to 'punish' those who had been seen to collaborate with the Germans, although some trials took place.

Sources:

Bernard Wilkin, 'Isolation, communication and propaganda inthe occupied territories of France, 1914–1918', First World War Studies, 7:3 (2016).

James E. Connolly, 'Mauvaise-conduite: complicity and respectability in the occupied Nord, 1914–1918', First World War Studies, 4:1 (2013)

James E. Connolly, 'Notable protests: respectable resistance in occupied northern France, 1914–18', Historical Research 88 (Nov 2015)

Jens Thiel, 'Between recruitment and forced labour: the radicalization of German labour policy in occupied Belgium and northern France', First World War Studies, 4:1 (2013)

If you're looking for a book about the occupation, then take a look at:

Helen McPhail, The Long Silence: The Tragedy of Occupied France in World War I (2014)

Let me know if you have any more questions!