Replace one or both of the commanders of First and Second Russian Armies, and improve their knowledge of communications. Remove the German staff officer (name escapes me) whose intimate knowledge of Russian military procedures (and the Samsonov-Rennenkampf feud) helped 8th Army to take advantage of Russian failings. Make the Germans less lucky at first, leading to a proper retreat even if Prittwitz is replaced. Austria and Russia stay the same as OTL. It's hard to do better than that!
Before the war France and the Ottomans conclude a secret agreement, which keeps the latter content and secure. So they don't ally with the CP. The war begins slightly later, as well, so the battleships are already in Turkish hands, preventing Churchill from nabbing them for the RN. The Brits are later than OTL in dealing with Ireland, and swifter at responding to the German invasion of Belgium, leading to two more BEF divisions than IOTL, and the whole force arriving days earlier.
The French have a different plan, recognising the possibility of German offensives through Belgium, and focusing on defeating these, then counter-attacking. They've also replaced their uniforms with a more modern version (several OTL proposals collapsed for various reasons. Just have one succeed). French heavy artillery has been under pressure since the efficacy of long-range fire was noted during the Boer War, and so by 1914 there's a lot more of it, and fewer 75s. Belgium is also a bit faster in re-equipping her forces ITTL, so has more of them (and better armed, trained and equipped) with which to face the invasion.
So Germany follows her OTL plan, runs into more stubborn Belgian defences, costing her more time and men than IOTL. By the time she penetrates it, she's facing a stronger BEF (pretty negligible, really, but it'd probably be ASB to have Haldane's planned million-man Territorial Force ready for the start of the war), French tactical doctrine is also sounder and equipment better. German casualties are greater than IOTL by several hundred thousand, and French lower by a similar amount - no constant counter-attacks to bleed them. German gains amount to Luxembourg and part of Belgium, while East Prussia has been abandoned to the Russians, and A-H has lost what it did IOTL. Serbia and so on is much the same. One could throw in Romania as immediately supporting the Russians, which could make the A-H losses even worse.
One
could throw in naval affairs - say one of the HSF's coastal raids on the UK goes awry, leading to the loss of several valuable warships. But while that sounds promising, and would have a negative impact on CP morale, there's potential for the German response to be to throw everything into submarine warfare, mucking up the British supply situation.
This amounts to a comprehensive failure of the CPs' plans, but still isn't as bad as things could get. I've concentrated here on the CPs' opponents doing better, but there's a great deal one can do to make the CP themselves (well, Germany mainly) worse.
