Ok, so I'm terrible at building stuff in the air in survival mode. It takes forever for me to place even one block because you need to be in such a precarious position so you don't fall, but I always end up falling and having to go all the way back up every few blocks.
Presumably there's nothing in the area below the farm that you would not want to destroy with spreading water, if so then simply take a water bucket and 8 blocks (dirt, cobble, whatever) to form a ring on a ledge near where you want to descend. Place the water in the center hole of that ring, give it a few seconds to flow far enough that you won't accidentally fall out of the water, and jump in. Move very slowly and cautiously outwards towards your building target from the middle of the stream until you lose the bubble bar, at that point use only the jump button to maintain your vertical position. You can practice this if you want, since it can be a bit tricky to smoothly maintain your position in a steady enough manner that you're not misplacing blocks all over.