Cut two pieces (8-9mm long) of the small PTFE that came with the thermistor. Slide the tube over the legs of the thermistor (separately).

Separate the legs of the thermistor as shown and insert the glass bead of the thermistor into the hole in the center of the heated bed. Solder them to the top two pads on the heated bed.

You can cut the excess leads after you have soldered it to the bed.