France: We want Alsace-Lorraine back
Germany: Alsace is German! The people there are Germans
Poland: We want Wielkopolska, Upper Silesia, and access to the Baltic
Germany: Never! That is integral German territory. (Ignore the fact that we stole it from you around 150 years ago.)
And Alsace-Lorraine had been stolen from the HRE 250 years ago.
The best would be to hold referendums in each county* of the disputed territories**. If a county has a population which wants to leave Germany, it will join France, Belgium, Denmark or Poland.
The same should have happened in Luxemburg, in Liechtenstein and in Austria to determine if these countries want to join Germany or to become independent.
Similarly, each county in Austria-Hungary should be given the choice to join Austria (Germany), Bohemia, Slovakia, Italy, Hungary, Romania, Serbia, Bosnia, Croatia or Slovenia.
Then, each country should have the right to enter a larher state like Yugoslavia or Czechoslovakia - or to opt out.
Finally, apply the same method to the Russian Empire, especially to determine the eastern border of Poland, and the borders of Ukraine and Finland.
This would be the best method to respect the right to self-determination of the nations.
But that shouldn't be the victory of nationalism. An effective League of Nation has to be created, to peacefully solve conflicts, and some kind of European Economic Organization set up to prevent major economic crises.
*City, district, Kreis, call it what you will.
**Alsace-Lorraine, South Tyrol, Sudetenland, southern Denmark, Eupen-Malmédy, Dalmatia, Upper Silesia, Danzig and Wielkopolska to name only the most notable ones.