1. Be sure the Interest rates are up  to date in Settings > Options > Interest rates

2. Be sure you know when the client will be paying the tax, just hoping they pay when you give them the return is not good. I generally don't like using the feature because I can't trust my clients to pay when I expect them to.

3. Know what type of return you want to do the computation for, as the screens are different for each each type of return. For the 1040 > Screen 8 > First item enter date discussed in #2, then > Late Return Penalty & Interest > use 1 for automatic. It will compute to the date you entered at the top of this screen.