OK - this really IS as simple as a good vinegar - any vinegar - some kind of flavours - spices, fruit, herbs... and giving them time infuse, or ferment a bit - and then add water.
You could make it simply directly into your glass with - apple cider vinegar - the same amount as you would cordial - and maple syrup - just for a bit of sweet - and then water. That's an easy one. Take that idea - and change it with different vinegars, different sweetner, maybe some fruit, fizzy or flat water.
Easy as that. I could write a book on more complex flavour combinations but you really don't need it. Here is one that has a few more ingredients.
NOTE - I often use frozen fruit - the flavour is imparted easier, and fuller somehow. Measurements are to be taken lightly. Use what you have and taste as you go.
INGREDIENTS
200g cherries, pitted, de-stemmed and halved
100g brown sugar/honey or raw sugar
1 cinnamon quill
dash of vanilla or a pod (you could use it again)
1 star anise
1 small mukrut lime leaf
a few sprigs of thyme or a bay leaf
250ml red wine vinegar or brown rice vinegar or ACV
1 tablespoon balsamic vinegar if you're fancy
EQUIPMENT
Jar or pot, funnel, muslin or small weave cloth, spoon, common sense
HOW TO
Pop all ingredients into a pot and bring to a simmer with the lid on for 15 minutes. I usually then let it sit overnight and sometimes a few days to ferment a bit and then bring to a quick boil before bottling. Keep refrigerated lest it explode!