Install ruby and stdbuf (most likely you already have them preinstalled)
Clone it somewhere, and optionally symlink watch_tablet into any directory in your $PATH
Copy a config file into ~/.config/watch_tablet.yml
Adjust the config (see below)
Add watch_tablet & to your ~/.xinitrc
Restart your desktop session and enjoy

input_device is a path to the device that provides the tablet mode switch. To find it you may run stdbuf -oL libinput debug-events|grep switch and notice something like event4 in the leftmost column. That would correspond to /dev/input/event4. Device numbers may be unstable across reboots, so you may consider doing ls -lh /dev/input/by-path and finding a symlink to that device. For X1 Yoga Gen2 it's /dev/input/by-path/platform-thinkpad_acpi-event.

modes.laptop, modes.tablet - this contain commands that will be executed when mode changes. Most likely this will contain xinput enable and xinput disable commands to enable/disable kb/touchpad/trackpoint (just run xinput to look them up). You may use any other commands to adjust your desktop environment (e.g. hide or show additional panels, increase button size, hide/show onscreen keyboard etc.)

:~$ stdbuf -oL libinput debug-events|grep switch
stdbuf: failed to run command ‘libinput’: No such file or directory

#! /bin/bash
#  sKeyboard
#  http://ubuntuforums.org/showthread.php?t=2100744
 
DEVICE="AT Translated Set 2 keyboard"
STATUS=`xinput list-props "$DEVICE" | grep 'Device Enabled' | sed 's/.*\([0-9]\)$/\1/'`
 
if [ "$STATUS" = "1" ]
then
    xinput set-prop "$DEVICE" 'Device Enabled' 0
elif [ "$STATUS" = "0" ]
then
    xinput set-prop "$DEVICE" 'Device Enabled' 1
else
    echo "Error : bad argument"
fi