Not the full solution, but some random thoughts:
Telling the leaders anything like "war will be long and bloody" will not work. They will not believe you, and you have no proof.
The only way I see is to let either side to win without world war, by breaking either Entente or Triple Alliance. Like, if British king or Russan tsar is killed by some French radicals.
Tell the truth about the devastation caused, if possible also to the public.
Germany ought to be more reluctant to attack with the knowledge that they lost hard in another timeline.
Tell them how much better EU-style cooperation is.
Suggest a NATO-style alliance.
If a Great War is started, promise to help the defenders by telling them everything.
My best path for a yes is through the mechanism of Great Britain being very explicit with Germany about their intent to abide by the 1839 Treaty of London.
For context, this is the one where the signatories promise to declare war on whoever invades Belgium, and was Britain's entry point into the war. There were at least some high ranking military officers who believed that had Britain said specifically that they would go to war if Belgium were invaded, Germany would have chosen not to invade.
OK, but if I am roleplaying the German side, I might choose to still start WWI but just not attack through Belgium. I will hold the Western Front with France and attack Russia.
Consider the following scenario: you are sent back in time some number of years before the beginning of WWI, with the goal of preventing the war. This includes preventing similar wars that happen slightly earlier or slightly later - you are aiming for a peaceful, stable coexistence between the European powers. Unsophisticated strategies like warning The Archduke against going to Sarajevo may simply result in another similar casus belli happening.
To prevent silly strategies, minor details of the world you are sent to will be changed, so dates and names may differ very slightly, but the overall WWI path will unfold almost exactly as it did in our timeline.
Furthermore, you carry some evidence of extraordinary status that is credible to the leaders of WWI nations. They will grant you an audience, but they will not blindly trust you. You may lie to them or tell them the truth.
You don't have detailed designs for post-1914 technology, but you have an ordinary informed layman's understanding of actual history. So you can tell them that nuclear bombs will be invented, but not exactly how, and they may or may not believe you.
Please answer with your strategy for saving Europe from World War I, and why you think it would work (including in the face of likely reactions from the major players)!