Pour the water in the jar into a pot and bring to a boil. Add the salt and sugar. Boil until everything is dissolved.
Take off heat and mix in the vinegar.
Once again, pour the water back into the mason jar. It is okay if it doesn't all fit!
Seal and flip upside down in a dark corner covered by a blanket overnight. This step is very important! It will slow down the cooling of the jar, so the tomatoes will process. The pressure of the upside down jar will allow it to seal. The main thing here is in Ukraine, you have different sealing equipment, so this seal might not be perfect. For this reason, once the jar is cooled (overnight, less than 24 hours), pop it in the fridge.
Leave in the fridge for 1 week if small tomatoes, and 2 weeks if the tomatoes are larger. Break the seal after 2 weeks and enjoy!