I would normally refer to Reddit, where I found my base recipe as well.. but we are now on lemmy!
Basically:
Make strong black tea, dissolve 70g cane sugar per liter. I think some people use less sugar maybe.
when cool, add to scoby from previous batch in large container with access to air through a cloth (prevents insects etc from getting in)
ferment for 1-2 weeks depending on temperature and desired tartness. This is F1
make syrup with fruits, sugar, water and strain or take some fruit juice
add to bottle that can withstand pressure (flip top bottles from Grolsch work well). Fill up with F1 kombucha. I think I use more or less 1:6 ratio of syrup:kombucha. Make sure to fill the bottle almost completely.
Ferment for another few days (I did 4 days now) in closed container to build carbonation. This is F2