Place hawthorn berries in a heavy-bottomed saucepan; add salt.
Add enough water to create about an inch at the bottom of the pan.
Heat until water and berries come to a boil; turn heat down and simmer for about 30-40 minutes, until berries have fallen apart. The juice in the pan will be thick and sticky.
Pour hawthorn berries and their juices into a jelly bag over a bowl to collect juices. If you don't have a jelly bag, line a colander with a thin dish towel or cheesecloth. Use a spatula or wooden spoon to gently press on the berries to help extract the juice. Allow to sit for about half an hour, or until juices stop dripping.
Remove jelly bag from stand and squeeze several times gently to extract as much remaining juice as possible. If using cheesecloth and a colander, bunch cheesecloth up into a bundle and squeeze.
Discard berry mash. Put hawthorn juice into a clean jar in the refrigerator until ready to use.