Then, he needs a way to neuter Britain for good. Like, somehow, the Royal Navy gets smashed, and England gets invaded, and then forced to demilitaritze itself for 30 years, relinquish all colonies and give independence to Ireland and Scotland, and possibly the French keeping bases/garrisons in several English coastal cities as well as London itself.
Because as long as Britain remains independent and a great power, it will keep encouraging other continental countries to make war against France, and the various states in Germany, Scandinavia, Italy and Iberia will always hope to regain their full independence (instead of forced economical and diplomatical alignment on France, or full-blown vassal status for some) because they'll know that Britain is there and ready to help. Which, inevitably, will lead to an endless cycle of wars (with maybe some periods of peace but again followed by another war).
The other condition for a stable peace in Europe (while the Napoleonic order is maintained) would be that Russia doesn't try to challenge France for the domination of the continent.
Which is possible, but requires that (1) again, Britain is completely declawed, so there's no hope or risk of Britain gold funding another Russian campaign, (2) the Russians are smashed during the alternate 1812 war, maybe instead of Napoleon campaigning into Russia proper, he manages to encourage a rebellion of Serfs across Russia, leading to a Russian civil war that collapses the Tsardom, and/or lures the Czar into campaigning in Polish and German lands, where Nappy has the advantage, and so Russia learns its lesson, and (3) Russia gets major concessions from Napoleonic France, such as a green light for taking over Constantinople and the Straits from the Ottomans as well as conquering Persia, and later expanding into China. And both Russians and French agree to keep any colonial conflict in Asia only (and never in Europe), and to always try to solve those colonial conflicts with talking first.
With the British and Russians out, the Spaniards, Portuguese, Dutch, Swedes and Danes aren't in position to resist French hegemony, nor are the smaller German and Italian states. And soon the Spaniards will need more and more French support to stave off rebellions in Latin America, as well as growing US (and possibly Brazilian) expansionism, while smaller Italian and German states will rely on French help against nationalists who want to create unified Italy and unified Germany (at the expense of the existing princes).
Prussians and Austrians might be slightly tempted to, but Prussians will also be encircled (with the pro-French Poles who really hate them on their backs, French bases in the Duchy of Warsaw, and possibly Swedish presence or threat in Pomerania). Or Prussia might even have ended up dismantled completely at the end of the Napoleonic Wars, after one coalition war too many. Austrians, in addition to be militarily crushed a few times, might be bought off with expansion in the western Balkans (possibly gaining Montenegro, Kosovo and Albania), and then will eventually be busy juggling all the restive ethnicities of their empire (and fear that, should a conflict arise with France, some ethnic rebellions will immediately start thanks to French support). Austria might also see France as the reason why Russia isn't tempted to export panslavism to the Croats, Slovenians, Czechs and Slovaks.