Use the sandpaper to remove the plastic coating from about 1cm of each end of the wire. You will see the silvery-copper wire colour underneath. Wind the wire around the large nail, but leave about 7cm at each end unwound. It will make about 80 turns. It does not matter if the coils lie on top of each other, though the tighter they are packed together the better. Push the coils towards the head of the nail so that the pointed end of the nail is exposed, then secure the coil onto the nail with pieces of masking tape.
Fold over each end of the sandpapered wire so that the pointy end of the wire is tucked away. Use a small piece of aluminum foil to cover the folded wire end. Fold the foil edges over to secure it on the wire. The foil will help to make the connection between the copper wire and the battery.
Hold the foil-coated wire ends over the ends of the battery to complete the circuit, then touch the pointed tip of the electromagnet against the smaller nails and paper clips.
When there is electricity from the battery flowing through the copper wire, the coils make a magnetic field, which is significant with the many turns. This magnetic field turns the large nail into a magnet, strong enough to pick up small nails, paperclips etc.
Break the circuit by removing one wire from the battery. The nail is no longer magnetic and it drops the objects. (Sometimes a weak residual magetic field remains in the nail, so a few objects remain attached.)
Note that this activity drains batteries very fast (there is not much resistance in the wire). The battery also becomes hot if it remains connected for a while so make sure it is disconnected frequently.

For more detailed explanation see www.wonderhowto.com/how-to/make-simple-electromagnet-326803/