You really can't use a normal switch, it needs to be listed transfer equipment but electrically it is a DPDT switch.
How much work/expense involved is mostly limited by how your panel is laid out now and how much rewiring you are willing to do. The easiest from the user/operation standpoint is to move the circuits you want backed up to a sub-panel with a transfer switch in it.
Some styles of main panels will take a transfer interlock device that works with your existing breakers but you will be responsible for your own load management.
If your main panel has a back fed breaker main directly across from a spare slot the cheaper interlock will work. When you start getting into interlocks for top mounted mains they start getting expensive, usually more than you can buy a transfer panel for.
