Help In Insert/update Multiple Selection From A Listbox

thank you in advance for all your BIG help. :)

I have 3 tables - SMaster, SI_map, ILookup

I need to create a form to Add/Update into SMaster and SI_map tables, which has one-to-many relationship, that's why I use a list box to show the values from SI_map.

in SMaster [ Sid, other fields ]
in SI_map [ primary key, Sid, ILookupid ]
in ILookup [ ILookupid, IName ]

questions:
1. How to get the multi-selected values from the list box?
2. insert these multiple records into SI_map table at the same time with SMaster (if I use the same form to populate all these fields)
3. how to populate the records into form for user to see and update the values?

Any suggestion is appreciated, it is very flexible to change any format such as SMaster and SI_map can be separate forms, as long as it works.

Thank you.