There were many people in positions of power in Germany that knew by the end of July 1918 that the war was lost (the Crown Prince of Germany was one of them).
I would say that 2 things happen vs. OTL:
1) Ludendorf is sacked after the black day at Amiens
2) when the Austrian leadeship visits Germany (as in OTL) August 15th 1918, saying that peace has to happen now or we are making a separate one, the Germans takes this seriously. At that point Germany + Austria issue a request via Holland expressing desire to seek a peacful resolution to the war, stating that the evacuation of France and Belgium to 1914 lines and the halting of submarine war could be pre-requisites to such a conference (thus the Allies know the Germans are taking this serious).
After a week the Allies agree but with extra terms: A cease fire will begin immediately. The Germans 30 days to evacuate France and Belgium, Germans have to move the entire German navy to the Baltic sea (Danzig), suspend construction of new naval vessels, aircraft, zepplelins, tanks and long range artillery, east africa will be evacuated, and allied observers will be allowed to monitor German military activities and factories. All Allied POWs will be returned in 60 days, all Central Powers enlisted men POWs will be returned in 120 days (all Central Powers officers will still be kept as POWs). Blockade will still be in place, but the Germans may be allowed to purchase food (but no guarantees). A conference will begin in one week in the Hague.
Italian, Balkans and Turkish lines will remain where there are (this pisses off the Italians and Serbians but this is about the British and French anyway), All Austrian and German naval vessels move to Pola however and all German crews must go to Germany thereafter.
So there you go: a peace conference at the end of August 1918, which gets the Germans out of France before its starts, but Germans still have some means of resistance however and thus are able to negotiate the worst parts of the OTL peace treaty. Any treaty will have to give AL back to france, give up all German colonies, have naval limitations, reparations (50% of Germanys 1913 military spending per year for 25 years), in exchange Germany gets a sphere of influnce in Lithuania or something.