the front is easy, a simple case of removing the badge (10mm bolt in the centre, and fishing wire to cut through glue), cleaning up the remaining glue, t-cut works, something like autoglym intensive tar remover works better.
then you have to cut off the locating pins on the back of the new badge with some cutters and then bolt it on (making sure it's the right way up)
TIP: the badge from the seicento fits better than the mk2 punto as there is no recess on the seicento...
rear ones are not bolted, however there are locating lugs on the back aswell as glue, so it leaves holes. remove in the same way as the front and fit the new badges, remembering to cover up the holes or you'll have a leaky boot.
EDIT: oh and welcome to the forum