Create a folder in /opt and cd to it
$ sudo mkdir -p /opt/syncthing
$ cd /opt/syncthing

Get the latest arm version of syncthing from github (https://github.com/syncthing/syncthing/releases/) x marks the version number.
$ sudo wget https://github.com/syncthing/syncthing/releases/download/vx.xx.x/syncthing-linux-arm-vx.xx.x.tar.gz
sudo tar -xzvf syncthing-linux-arm-vx.xx.x.tar.gz
sudo rm syncthing-linux-arm-vx.xx.x.tar.gz

Move content one folder up
$ sudo mv syncthing-linux-arm-vx.xx.x/* .
$ sudo rm -r syncthing-linux-arm-vx.xx.x

link to it (not moving) so you can call it using 'syncthing'
$ sudo ln -s /opt/syncthing/syncthing /usr/bin/syncthing

Start syncthing
$ syncthing

Wait until you get something like:
INFO: Device 7NNIJMJ-RQ657WA-RI5YH6L-RQ657WA-RQ657WA-VGKSUYP-U6QBJNA-RQ657WA
write down this number or copy it if you SSH'd to your pi.

quit the operation using CTRL + C.

Change the config file so you can access the GUI from any PC within the network
$ nano ~/.config/syncthing/config.xml

Change

<gui enabled="true" tls="false">
    <address>127.0.0.1:8384</address>
</gui>

to

<gui enabled="true" tls="true">
    <address>0.0.0.0:8384</address>
</gui>

OPTIONAL: if you forward port 8384 (or any port you would like to use) on your router to you pi, you can access the GUI from anywhere in the world as long as you know you home(public) IP address.
Make sure you use https and enable authentication!

Start syncthing again
$ syncthing

on a PC in the same network, go to the GUI and disable automatic updates.
GUI -> Settings -> Disable Automatic updates
hit save and restart syncthing.

To enable syncthing on boot

cd to /etc/init.d/
$ cd /etc/init.d/

Download the script(Hint: its the script below)
$ sudo wget https://gist.githubusercontent.com/OIWitteveen/3452618e182ce9746343/raw/4cc25a607197f0d58ae8e416a986795cce9cd17e/syncthing

Change the directory line in the script to match the syncthing location
$ sudo nano /etc/init.d/syncthing

Change

DAEMON=/usr/local/bin/syncthing

to

DAEMON=/opt/syncthing/syncthing

exit and save (CTRL + X -> ENTER)

Make the script executable
$ sudo chmod +x /etc/init.d/syncthing

make the script start during boot
$ sudo update-rc.d syncthing defaults

Restart the the pi to apply changes
$ sudo reboot

If there's a new version of syncthing, use
$ sudo syncthing -upgrade
Then reboot the system.
$ sudo reboot



To remove syncthing use
$ sudo rm -r /opt/syncthing
$ sudo rm /usr/bin/syncthing
$ sudo rm -r /home/pi/.config/syncthing
$ sudo rm /etc/init.d/syncthing

optionally, delete any folder used with syncthing.