How do you install an internal floppy disc drive into a laptop?

Most modern laptops do not have space for a floppy drive internally, so this is impossible on them.

If you have an old laptop that has space for a floppy drive internally, it probably already has one so you don't need to install one. However if that existing internal floppy drive has failed you will have to replace it (assuming you can find a floppy drive to replace it with). To replace the existing internal floppy drive you will need the manufacturer's documents on how to open the laptop's case, get access to the floppy drive, and swap out the old one and replace it with the new one. This will almost certainly require special tools (e.g. torx drivers, wrenches) that you don't already have and will need to buy if you don't already frequently do such work.