During reassembly, I found it easier to insert the ribbon from the new fan while the fan was loose and held in my hand. Once the ribbon was inserted and locked down, then I seated and screwed in the fan.

I found it much easier to reinsert the ribbon while the reader board was fully disconnected. This way I was able to simply hold the reader board with my fingers and slide it over the end of the ribbon while the ribbon was just sticking up in the air. The ribbon is quite stiff and slipped in VERY easily in such a way that I felt confident that it was fully seated. Then while holding it in one hand, I flipped the locking flap down with a spudger in the other. You might also be able to seat the board and the ribbon will likely stay inserted under its own tension, then flip the locking flap down.

You can use a spudger to hold down the black plastic side of this connector that is supposed to stay attached to the motherboard while using the pointy spudger as shown in order to reduce any chance of pulling the socket off the motherboard.