Always update your PI before doing anything else

sudo apt-get update
sudo apt-get upgrade

Install transmission

sudo apt-get install transmission-daemon

Change default directories where torrents will be stored (default one is hard to find)

sudo mkdir -p /home/pi/torrent

Change directories, username, password and IP whitelist in transmission settings file

sudo vim /etc/transmission-daemon/settings.json

    "download-dir": "/home/pi/torrent", 
    "incomplete-dir": "/home/pi/torrent", 
    "queue-stalled-enabled": true, 
    "rpc-authentication-required": true, 
    "rpc-bind-address": "0.0.0.0", 
    "rpc-enabled": true, 
    "rpc-host-whitelist": "", 
    "rpc-host-whitelist-enabled": true, 
    "rpc-password": "your_password", 
    "rpc-port": 9091, 
    "rpc-url": "/transmission/", 
    "rpc-username": "pi", 
    "rpc-whitelist": "127.0.0.1,192.168.*.*", 
    "rpc-whitelist-enabled": true, 

Restart transmission to apply the changes

sudo service transmission-daemon reload

Stop transmission deamon and change user and permissions

sudo service transmission-daemon stop
sudo vim /etc/init.d/transmission-daemon

NAME=transmission-daemon
DAEMON=/usr/bin/$NAME
USER=pi
STOP_TIMEOUT=30

Change owner of transmission deamon files

sudo chown -R pi:pi /etc/transmission-daemon
sudo chown -R pi:pi /etc/init.d/transmission-daemon
sudo chown -R pi:pi /var/lib/transmission-daemon

Change user in transmission-deamon.service file

sudo vim /etc/systemd/system/multi-user.target.wants/transmission-daemon.service

[Unit]
Description=Transmission BitTorrent Daemon
After=network.target

[Service]
User=pi
Type=notify
ExecStart=/usr/bin/transmission-daemon -f --log-error
ExecReload=/bin/kill -s HUP $MAINPID

[Install]
WantedBy=multi-user.target

reload the systemctl

sudo systemctl daemon-reload

create the directory where the setting.json file will be accessed

sudo mkdir -p /home/pi/.config/transmission-daemon/
sudo ln -s /etc/transmission-daemon/settings.json /home/pi/.config/transmission-daemon/
sudo chown -R pi:pi /home/pimylifeup/.config/transmission-daemon/

Start transmission service

sudo service transmission-daemon start

Access your PI torrent box from a web browser

http://your_pi_ip:9091