Download:

https://sourceforge.net/projects/dban/files/dban/dban-2.3.0/dban-2.3.0_i586.iso/download

write it onto a spare USB (https://rufus.ie/en_IE.html) or a spare CD.

Once booted into the USB follow the prompts, it's super simple and intutive.

It could take up to 24 hours depending on the hard drive speed to do the wipe.

I highly recommend however to just remove the hard drive entirerly and just sell it without the HDD. It is much more secure and less risky of data recovery. Worth reducing the sell price by $20 then risking idenity theft.