From what I remember when I was researching my own situation, the authorities usually want documents that clearly connect you to the ancestor who was a Lithuanian citizen, so things like birth certificates for each generation, marriage certificates if surnames changed, and sometimes records proving when that ancestor left Lithuania. It sounds like a lot at first, but once you start mapping the family tree it becomes easier to see what’s missing. I found a helpful explanation of the typical paperwork and steps while browsing https://www.lithuaniancitizenship.com/ — it breaks down the process and the kinds of records people usually collect before applying. In my case, I began by asking older relatives for any documents they kept in drawers or photo albums, and surprisingly we found several useful papers that helped confirm the family line.