During reassembly, in order to reconnect the fingerprint sensor cable, first angle the back cover into position until the cable connector lines up perfectly over its socket.

Then, use the flat end of your spudger to gently snap the connector into place by pressing it straight down.

If you have slender hands, you may be able to press the connector into place with your finger. Just be careful not to strain the cable.

This takes patience and a bit of practice. Don't rush it or attempt to force the connector into place.