If you are a LA resident, but work in MS, you file a MS Non-resident Tax Return and a LA Resident Tax Return.
You will be taxed by MS for only the income earned in MS. Your state of residence (LA) taxes you on your world-wide income.
You receive a credit on your LA tax return for taxes paid to another state so you are not taxed twice on the same income.
It is important that you do your MS tax return first so the income and credit post through correctly.
If you have already started the LA tax return, delete it and complete the MS first.