To use a netboot install medium (I'm assuming something much like mini.iso?) it'll be necessary to have a wired internet connection. If this isn't feasible, download the desktop USB image and use that for the install. Frankly I'd suggest doing that anyway, as it's rather more user-friendly.
As for the actual process: Ensure the laptop can boot from USB. Install the new hard drive. Insert the bootable USB. Power the machine on and access the boot device selection part of the BIOS menu. Choose to boot from the USB stick. Install as per the instructions. That ought to be enough, but for more specific advice do come back here!
The normal Ubuntu download page (and the best place to get a desktop image) is here, and instructions on its use can be found here