heres my howto that i made for myself, with some amendments. you may have to edit the paths etc. someone with better knowledge may have better ideas or corrections, but it works for me
first make an executable script, lets call it startup.sh (as its gonna run at startup, you can later add more commands that u may wish to run at startup as well). personally i placed mine in /opt (you may have to make this folder)
in startup.sh you place the commands you want to run, one per line
then you need to make a systemd service file
make a text file, call it startup.service, and it should look like this
[Unit]
Description=Run Script at Startup
[Service]
Type=oneshot
ExecStart=/opt/startup.sh
[Install]
WantedBy=multi-user.target
then you need to copy the starup.service to the correct folder
sudo cp /path/to/startup.service /etc/systemd/system
then you need to enable the service
sudo systemctl daemon-reload
sudo systemctl enable startup.service
check if its enabled: systemctl is-enabled startup.service