There is some information on the thread "missing prop" put it into the search box.
The things to remember are,
1 use loctite 243 on the threads.
2 use a block of wood from prop blade to the hull so you can get purchase on the bolts.
3 the instructions that are in your Volvo book are good and easy to follow, if replaceing fixed with a folding Volvo prop they again are easy to follow and the new type of Volvo prop are better designed to stop them falling off, as the locking nuts and tags are much better.
Loosing a prop on a 120 leg is not uncommon, mainly because no loctite was used and the bolts were not tightened correctly.
Google Volvo prop fitting procedure.
Hope this helps
Odysseus.