Just try a bunch of techniques until you find one that feels right. I stream like I am playing the piano, but there are so many different techniques and no one is the same. You can try index and ring finger, or super curled with middle and ring finger, although I don't like this method because it puts a lot of unneeded strain on your knuckles. Another technique is keeping your fingers outstretched, but you need to build up a lot of stamina if you want to use this method.
In terms of building up stamina, just play longer songs. Play 5+ min songs all of the time. It doesn't matter if you get B's or C's or that you won't get any pp, you will be better off in the long run. Even if you feel your hand get tired while playing, don't pause. Your fingers speed and stamina relies on the muscles, so by playing longer songs to the point where your fingers start to burn a little is a good thing. It is just like a regular workout. You wouldn't get any faster at running if every day while on a mile long jog, you stopped and took a 2 min break 3 times during the mile. You need to commit to playing longer songs with streams. It will suck at first, and your hand and forearm may actually be sore sometimes if you play long enough.
If after committing to playing stream heavy maps like Haelequin (extended version obviously) or maps with lots of 5 note bursts like Blue Dragon or Ultramarine (both Seiryu songs btw), and you still don't notice any improvement after around 3 weeks, then you may want to try tap/x playstyle, as you won't get as tired from streaming since two hands are doing the work.
Oh, also one more thing. You should really be able to start streams from either finger. If you always start streams with your middle finger, start training yourself to be able to streams from index finger, or vice versa. This is really helpful so you don't rush to start streaming with your preferred finger if the previous stream ended on your preferred finger. You can instead just start with the finger that wasn't used to hit the last note of the previous stream.
Good Luck, but don't hurt yourself either! (well maybe just a little;})