I too had no idea what I was doing when starting out and two years later I am pretty confident with where I am. I am just playing whatever songs I feel like it 99% of the time and have been that way the whole time. I have definitely learned quite a lot and would consider myself at or near an intermediate level (I can do most rhythm parts with little to no problem and can do some less shreddy solos with adequate success). I really never really considered myself to follow any sort of syllabus. I always just hop on, play and have fun, then stop when I feel like it. If you feel like you need one though, I would recommend just aim to play a certain song/section to a level you deem satisfactory. Some of the best RS advice I have heard is to disregard the percentage and shoot for a sound that you like. If you get a 98 but it sounded good to you, then I would count that as a win. There really isn't much of a right way to do it, just do it to your standards. If you're on PC, I would highly recommend downloading OBS or some other free recording software and recording yourself when you play. It's really fun to hear yourself playing after the fact and can better help you judge where you are and if you're happy with where you are or if you want to practice a certain part more. I would also recommend just playing what you want to play. In the early days for me it was fun to play the songs I liked regardless of how low of a difficulty I had it set to. Lastly, do try out rhythm mode because sometimes the chart is different and a new experience and it usually doesn't force you to sit out a solo if you aren't up to it yet. Maybe give bass a go too whether it's the simulated bass or on a cheap bass because that too is really fun and can help build you up.