It's easy peasy. Just slice the tomatoes and pour enough vinegar over to cover them without over saturating. Sprinkle with a little sea salt and black pepper.
I think these Balsamic Marinated Tomatoes taste the best after marinating for 24 hours.
Nutritional information depends on the brand of balsamic vinegar used. If you're keto, and you'd like to cut back on the sugar even further while adding some fat, you can do 1 part balsamic vinegar and 1 part olive oil.