What NameTaken said is proper way to do it, except user services will be inside ~/.config/systemd/user/service.name. example (change user/group for your username, unless you need to xmr-stak running as root then place it inside /etc/systemd and don't specify any)
$vim ~/.config/systemd/user/xmr.service
[Unit]
Description=xmr-stak
After=network.target
[Service]
User=user
Group=user
Type=simple
RemainAfterExit=yes
ExecStart=/path/to/your/xmr-stak -c /path/to/config
[Install]
WantedBy=default.target
$systemctl --user enable xmr.service
ps: you can add tmux/screen if you need it -
ExecStart=/usr/bin/tmux new-session -d -s xmr /path/to/your/xmr-stak -c /path/to/config