Without guarantees of France and UK. (Example, any war of Balkan nation with another Balkan nation)
In the regular game, guarantees cannot be issued until 25% WT. Iirc, the allies guarantee any democratic or non-aligned country that is being threatened by a fascist or communist country.
Okay, so a good approach would be to not join a faction and try to DOW before say mid 1937 ?
Yes, assuming the nation you are targeting doesn't already have a guarantee.
If you can keep your contribution to World Tension below 10%, you can also get away with two justifications at the same time. Pick 3 states for the first justification, then a single state for the 2nd justification. That will allow them to finish within 40-50 days of each other. Since your contribution to world tension won't spike again until you declare war, wait until you have two war goals available, then declare war on both nations at the same exact moment. Once you win those wars, you should be able to annex both targets without having to fight any majors.
Of course, this is very generic advice. If you have more information about which nation you're playing, and who your targets are, more specific feedback can be given.
Communist Yugoslavia vs Bulgaria and Romania.
Does Yugoslavia lose its French guarantee when they turn communist?
If not, that guarantee balances the one to Romania and France will remain neutral. Similar to the way American minors can wage war on each other early and USA stays out of it.
Does Yugoslavia lose its French guarantee when they turn communist?