All you have to do, really, is switch the First Battle of the Marne and the Battle of Tannenberg. The Marne is somewhat easier; just have the famous taxis carrying 6000 soldiers not reach the front in time and you most likely have a German victory. This allows the Germans to keep up the momentum in the West and most likely take Paris. While that would be a massive blow to French morale, I tend to doubt they would surrender. Most likely trench warfare still develops west of Paris and the WF remains a, albeit more successful and eventually victorious, manpower hole for Germany.
The East is a bit different. Tannenberg was lost not just due to superior German supply, but also a lack of effective communication between Russian armies, due to technological problems and personal animosity between Rennenkampf and Samsonov. The first and most important thing is to replace Samsonov. While inarguably brilliant, he was untested and difficult personality-wise. The second thing is to fix Russian communications; better ciphers, a more extensive telegraph system, probably other things I haven't thought of. With both of these together, the 8th Army is most likely forced to retreat, if not destroyed, by superior Russian numbers. Then you have a huge advancing force in Ostpreussen and some serious forward momentum for the Russians.