For the Central Powers to win (in a way we can reasonably historically extrapolate from), submarine warfare as conducted by Germany would have to remain restricted, and the telegraph the Germans sent to Mexico (telling them to attack the U.S) would have to either not be sent or not be intercepted by the British. With this, American interference would be stalled for a while, possibly until the end of the war.
The war itself would change with Russia's capitulation. At this moment, as in our history, the Russians would officially cede Poland and most of Ukraine, losing a lot of what was left of both their industry and agriculture. Russia is effectively neutered in this timeline, as stated above. The battered German, Ottoman, and Austro-Hungarian armies might revolt or demand rest after fighting on the Eastern front, but in the Western front, morale effectively collapses for the French, and soars for the Germans
With the loss of their Eastern ally (and no U.S to assist), mutinies would spread through the French trenches like wildfire, and the Germans, emboldened by their victory in the East, would likely send a massive counteroffensive. This would cause those few French that didn't surrender immediately to capitulate under the force, as the Eastern troops finally arrive two months later. Paris is encircled and seized after a short, brutal city battle, and the UK's forces withdraw across the Channel.
We have two possibilities from here. A British pseudo Cold War as the UK and Germany stare at each other from across the channel, while bitter fighting happens in the colonies (somewhat similar to the real Cold War with fighting in Afghanistan, Vietnam, and Korea. Just on a larger scale.), to which, I do believe the Germans and Ottomans would gradually force the British out. This would take another five years, however, assuming the British and Germans don't do what I consider the more likely scenario. I will no longer extrapolate in the Continued War possibility, because I authentically don't know that much about the fighting in that area.
The British, well aware of the bloodshed and probable loss of several colonies, would accept German terms, (really just throw France under a bus), losing very little. The war ends around February, 1919. Casualties are about two million greater than in our timeline.
Now, a period of instability occurs in Africa, Russia, Belgium and France. With Africa, the changing of ownership would be slow and messy, French and Belgians resisting at every opportunity. But without mainland France or Belgium, their rebellions are crushed one at a time. Life is very similar for the natives, so they don't take any one side in general. In Russia, Belgium and France, anger over their loss and the humiliating terms they had to agree to leads to resentment and hatred. In Russia, the Reds overtake the White army, as the German force would still be recovering from the Great War, and the government would be more interested in affairs closer to home. In France and Belgium, civil war rages for months, and the Germans intervene for the factions most useful to them. Nevertheless, the war takes months to resolve. A decent portion of Germany's gains are basically useless, but the rebels do not gain sovereignty. Resentment builds in the west.
In Germany itself, the November Revolution's first phase occurs, but the second ends in February with German victory. The Kaiser remains in power, using their victory as a means to cling to power and as a result, the Wiemar Republic does not form. This blocks Hitler from forming the Nazi party at all, as there would be no need for it. Hitler himself, however, along with all of the veterans of the War, now face the problem of reintegrating into society. Germany's economy is still damaged by the war, and looting the remnants of Russia simply patches the holes. But for now, it remains stable, if a little shaky. Over time, the Kaiser would gradually see the homeland rebellions drop off as he focuses on turning their African colonies into successful endeavors.
Italy, however, nearly collapses as the furious Central Powers focus their fire on it after the fall of France, seizing much of Veneto, Trintina and South Tyrol. Mussolini's March on Rome occurs, possibly earlier than normal, and Italy now awkwardly attempts to endear itself to the Central Powers.
Japan maintains the fight for a little while, but soon comes to terms with the Central Powers, ceding much of the land they gained. They focus on the weakened China and Russia, carving swathes of territory for themselves. Manchuria, Korea, and possibly even parts of Sibera are now Japanese. With Russia basically obliterated, they slowly start marching west, but don't get too far, the frozen wastes of Siberia far more effective than the shattered Red and White armies. They seize enough, however, to start really trading with the Central Powers, possibly overland, if not, then around the Cape of Good Hope.
In France, revolution after revolution occurs until a French super-nationalist party claws its way to the top, using the message of reclaiming French territory. Similar revolutions occur in Belgium, and these paramilitaries are sheltered in Britain, who is still nominally against the Germans. Over time, this rebel force trains itself and grows through discreet recruitment in the homelands.
World War II does not happen the way we know it, and monarchies remain in power. Gradually Japan would expand its sphere of influence, possibly annexing most of China in time. The U.S.A starts trading with the Central Powers, and although the Spanish Influenza pandemic strikes hard, the world remains stable.
For a time.
As an addendum the Ottomans and Austro-Hungarians still stumble on, although Lawrence of Arabia makes things irritating for the Sultan. The Sick Man of Europe still is slowly dying, and Germany and Austro-Hungary are eager to carve their old ally to pieces.
