The easiest way to get the most drawn out war would not to have Hindenburg-Ludendorff. This means no Americans and a better situation at home without the Hindenburg Program. So Germany has no riots, launches several limited offensives on the western front, nothing like Kaiserschlacht. These inflict casualties while preserving German offensive power and fortified positions. Pretty much spoiling attacks.
While Germany is hanging in there, Austria-Hungary collapses and turns into a gigantic clusterfuck that no one wants to touch, and Germany is forced to intervene only to secure its borders and probably annex the German speaking portions.
Bulgaria collapses, as do the Ottomans, meaning that the British have new manpower to use despite occupation duties. The French are wearing down, as are the Germans. But the Germans can hold out until now until 1919 (variety of factors that I won't get into detail with...unless you really want to know). That means Germany+Austria are resisting the Italians at the Brenner Pass, though the Italians have just rounded up 400,000 Habsburg soldiers and are digesting them. Austria is starving and probably is draining Germany's resources. Hungary goes communist, but is left alone by the Germans. Romania recovers with allied help, but now gets sucked in to fighting the Hungarians under Bela Kun. Serbia goes on creating Greater Serbia and may come into conflict with the Italians who are trying to occupy the Dalmatian coast. The Greeks and Turks are going to go at it eventually, but probably not until 1920.
The Russian Civil War is going on and probably will see active allied intervention in the Crimea to make it solidly White Russian and deny the Germans grain in 1919. Maybe landings at Murmansk too. Allied intervention in Hungary, which means that they are going to get sucked in the fighting there and probably the need to feed the starving populations.
Poland and the Baltics are German, and may start producing food for Germany, but at the expense of the locals, meaning large starvation among non-Germans. Perhaps uprisings in the East as a result. I doubt the Germans can afford to keep people as deeply in the East as OTL. Caucasia gets abandoned and Western Ukraine is the only part sort of occupied.
The situation is ugly, as the British have a number of commitments for their newly-freed up troops from the Middle East, but they are then able to try and attack Germany's flank through the former Habsburg empire, only to find that transportation (trains) are not there to help them. Plus the lines are falling apart. Plus there are several civil wars to get through.
If the Germans can get the Austrians to play along, they can hold up the Italians at the Brenner pass, easily. But they are going to have to fight the Czech for control of the Sudeten, which may not be too hard given that so many of their fighting men are now in Russia. Come 1919 though, the Czech legion and their 67,000 men are going to make an appearance in the Balkans, as they get shipped out of Russia. Of course, sustaining them in the Balkans is going to be difficult once they get beyond the beachheads. Maybe the war ends 1919 or 1920 when the Allies (French) have had enough. Or when the Germans finally did. Or....