Put your empty, clean glass jars into your big pot of water. Turn on the stove and let the water get hot and simmer. This warms up the glass so it doesn't shatter when you pour hot liquid into it later.

Use tongs to carefully pull the hot, empty jars out of the water and set them on a towel. Drop 1 garlic clove, dill, and a pinch of red pepper flakes into the bottom of each jar. Then, pack your raw cucumber slices into the jars as tightly as you can.

In a separate cooking pot on the stove, mix the 3 cups vinegar, 3 cups water, and 2.5 tablespoons of salt. Bring it to a boil. Stir it until the salt completely dissolves and disappears.

Carefully pour that boiling vinegar liquid into the jars over your cucumbers. Stop pouring when there is 1/2 inch of empty space left at the top. Wipe the glass rims clean with a damp paper towel. Put the flat lids on, and screw the metal rings on until they are comfortably snug.

Put the filled, closed jars back into your big pot of boiling water. Make sure the water covers the tops of the jars by at least an inch. Let them boil completely for 10 minutes.

Lift the jars out of the water and set them on a towel to cool for 24 hours. As they cool, the air will escape, and you will hear a loud "POP" - that means the jar is officially vacuum-sealed and safe to store on your shelf!