At the end of /boot/config.txt (replace 21 with pin number):

dtoverlay=w1-gpio,gpiopin=21

Save and reboot.

ls -l /sys/bus/w1/devices

Grab its address (format 28-xxxxxxxxxxx). Read the temperature with

cat /sys/bus/w1/devices/28-00[address]/w1_slave

The numerical value after t= is the temperature in thousandths of a degree C.