In order to start a Linux process on the background, just put & at the end of the command
For example, to run a sleep command at the background, you do the following
sleep 10000 &
The above will sleep for 10s at the background.
When this command being executed, it will return a PID for the background process.
So, to kill the background process, you can use kill command to kill the background process
kill -09 XXXX
where XXXX is the PID returned for the background process.
Below is a detail screenshot.